Supported biofilm process

ABSTRACT

A membrane supported biofilm reactor uses modules having fine, hollow fibres, for example, made from melt spun thermoplastic polymers treated after spinning to increase their permeability to oxygen, used, for example, in tows or formed into a fabric. In one module, one or more sheets of the fabric are potted into a module to enable oxygen containing gas to be supplied to the lumens of the hollow fibres. Various reactors and processes, for example to treat wastewater, using such modules are described. In one process, oxygen travels through fibers, optionally through an attached biofilm, to oxygenate surrounding water. Mechanical, chemical and biological methods, for example endogenous respiration, are used to control the thickness of the biofilm.

This is (1) a continuation of U.S. application Ser. No. 11/203,197 filedAug. 15, 2005; which is a continuation-in-part of InternationalApplication No. PCT/CA2004/000206 filed Feb. 13, 2004 (2) a continuationof International Application No. PCT/CA2004/001495 filed Aug. 12, 2004;(3) a continuation-in-part of U.S. application Ser. No. 10/896,086 filedJul. 22, 2004; (4) a continuation-in-part of U.S. Ser. No. 10/801,660filed Mar. 17, 2004, now U.S. Pat. No. 7,169,295 which is an applicationclaiming the benefit under 35 USC 119(e) of U.S. Provisional PatentApplication Ser. No. 60/496,178 filed Aug. 18, 2003; (5) acontinuation-in-part of U.S. Ser. No. 10/777,204 filed Feb. 13, 2004 nowU.S. Pat. No. 7,118,672 which is an application claiming the benefitunder 35 USC 119(e) of U.S. Provisional Patent Application Ser. No.60/447,025 filed Feb. 13, 2003; (6) a continuation-in-part ofInternational Application No. PCT/CA2004/001496 filed Aug. 12, 2004;and, (7) a continuation-in-part of Ser. No. 10/895,959 filed Jul. 22,2004. This application also claims priority from Canadian PatentApplication Nos. 2,438,444; 2,438,441; 2,438,432; 2,438,050; and,2,438,101 all filed Aug. 22, 2003 and Canadian Patent Application No.2,458,566 filed Feb. 13, 2004. All of the applications listed above areincorporated herein in full by this reference to them.

FIELD OF THE INVENTION

This invention relates to a gas transfer apparatus and process, forexample to support a biofilm in a liquid, as in a water or wastewatertreatment process or apparatus and more particularly to a membranesupported biofilm module and process.

BACKGROUND OF THE INVENTION

Currently, most wastewater treatment plants use an activated sludgeprocess, based on biological oxidation of organic contaminants in asuspended growth medium. Oxygen is supplied from air using bubble typeaerators. Efficiency of these systems is poor resulting in very highenergy use. Tank size is large since oxygen demand loadings are low. Theresult is high capital and operating cost.

A second type of established biological oxidation process uses biofilmsgrown on a solid media. For example, the wastewater may be circulated tothe top of the reactor and trickles down. Air is supplied at the bottom.The rate of oxygen transfer is limited by the biofilm surface area, andthe operating cost is high because of wastewater pumping requirements.

Recently, development work has been done on a membrane supportedbioreactor concept. For example, U.S. Pat. Nos. 4,181,604 and 4,746,435describe a process for treating wastewater by supplying oxygen from oneside of a gas-permeable membrane to micro-organisms growing on the otherside of the membrane. Hollow fibers with porous walls were used as themembrane. In U.S. Pat. No. 5,116,506, a gas permeable membrane divides areactor vessel into a liquid compartment and a gas compartment. Abiofilm is grown on the gas permeable membrane on the liquid side of themembrane. Oxygen and alternate gases pass through the membrane to thebacteria growing on the liquid side of the membrane.

SUMMARY OF THE INVENTION

It is an object of this invention to improve on the prior art. It isanother object of this invention to provide methods and apparatussuitable for treating water, for example industrial and municipalwastewater, using membrane supported bioreactor technology. It isanother object of this invention to provide a hollow fibre gas transfermembrane and module which is, for example, suitable for supporting abiofilm. It is another object of the invention to select a membranematerial suitable for use in membrane supported biofilm modules andprocesses. These aspects and others are met by the invention describedand claimed herein. The following summary will introduce the reader tovarious aspects of the invention but is not intended to define theinvention which may reside in a combination or sub-combination ofvarious elements or steps found in the following summary or other partsof this document.

In one aspect, the invention provides a membrane and module with areasonably high gas transfer rate and adequate surface area, for oxygentransfer, biofilm support or both, to allow a membrane supported biofilmreactor to provide an operating cost advantage over other processes usedin the art. The membrane and module may have an oxygen transferefficiency (OTE) of over 50% or in the range of 50% to 70% or more. Themodule may be made of non-porous or dense walled hollow fibre membranesto provide a large surface area while avoiding the tendency of porousfibers to wet over time which results in a drastic drop in their oxygentransfer rates.

In another aspect, the invention provides a very fine dense hollow fibremade from poly methylpentene (PMP), which has a high selectivity anddiffusion coefficient for oxygen. In particular, PMP has a gaspermeability of about 70,000 cc·mm/m²·24 hr·Bar in dense wall,non-wetting form without being treated to increase the porosity of thebase material. While this is significantly less than silicone, which hasan extremely high gas permeability, PMP may be melt spun into a hollowfibre. The fiber can have an outside diameter of 500 microns or less or100 microns or less. Use of such a small diameter fibre helps reducemodule cost as textile fine fibre technology can be used to createmodules. A very large surface area can be provided to achieve high OTE.The non-porous wall, meaning a wall with only very small openings, forexample of 30 Angstroms or less, or about 4 or 5 Angstroms in the caseof unmodified PMP, avoids wetting which would reduce the flux throughthe fibers.

In another aspect, the invention relates to an apparatus or processusing other than homogenous dense walled fibers. Buy using PMP in hollowfibre form, an effective module can be made. However, PMP is stillexpensive compared to more common materials. For example, it iscurrently about 10 times as expensive as other thermoplastic polymersincluding more common polyolefins such as polyethelene (PE) andpolypropylene (PP). These substances have poor gas transfer rates indense wall form of only about 4,000 to 8,000 cc·mm/m²·24 hr·Bar. Thesematerials may be made in a microporous form with much increased gaspermeability and treated chemically to make them hydrophobic. However,the chemically treated pores tend to wet out in practice when immersedin water containing surfactants or proteins resulting in a significantdecrease in flux rate. Alternately, the fibres may be melt spun to havea partially dense, asymmetric, variable porosity or homogenous wallwhich does not permit water flow even without chemical treatment to makethe fibers hydrophobic, but has gas permeability over the standardvalues mentioned above. The increased permeability results from alteringthe method used to make the fibres, for example by thermal or physicaltreatment after the fibers leave the spinneret. The method of making thefibres also typically reduces the selectivity of the membranes, forexample selectivity to carbon dioxide or nitrogen (O₂/N₂, O₂/CO₂) may beas low as 5 or less for PP or 3 or 1.5 or less for all fibers. This lackof selectivity may make the fibres unsuitable for gas separation but theinventors have found that high selectivity is not required, and may evenbe undesirable, in many water treatment applications. In particular,where air is used as a source of oxygen to support a biofilm growing inwater, there are minimally adverse or even useful partial pressuregradients of carbon dioxide or nitrogen across the membrane wall. Thepolymer used for the fibres may still be a highly gas permeable polymersuch as PMP. However, the fibres may also be made of less expensivepolymers, for example other polyolefins such as PE or PP. The polymer isextruded as fine hollow fibres by melt spinning under certain conditionsand subject to post-treatment whereby the permeability to gases isincreased, for example to 20,000 or 30,000 cc·mm/m²·24 hr·Bar or morefor a PE or PP fibre.

In another aspect, the invention provides a fabric with a very largenumber of hollow fibres, for example of any of the fibers as describedabove, providing sufficient surface area so that oxygen transfer doesnot become a limiting factor in controlling biological kinetics. Thefabric may be made, for example, with the hollow fibres, optionallycollected into units, woven as weft and an inert fibre as warp tominimize the damage to the transfer fibre while weaving. Other methodsof preparing a fabric may also be used. The fabric provides strength tothe fine fibre to permit biofilm growth on its surface with minimalfibre breakage.

In another aspect, the invention provides a module built from fabricsheets with very high packing density to permit good substratevelocities across the surface without recirculation of large volume ofliquid. The modules enable a supply of oxygen containing gas, such asair, to be supplied to the lumens of the hollow fibres without exposingthe lumens to the wastewater. Long fibre elements, for example between 1and 3 metres or between 1.5 and 2.5 metres are used and potted in themodule header to provide a low cost configuration.

In another aspect, a biofilm is grown on a fabric made from a gaspermeable hollow fibre, for example any of the fibers described herein.Oxygen bearing gas is introduced into the lumen of the fibre. Aerobicreactions take place near the surface of the fibre, where the highestlevels of oxygen exists. These reactions include conversion of organiccarbon compounds to carbon dioxide and water, and ammonia to nitrates.The surface of the biofilm is maintained under anoxic conditions suchthat conversion of nitrates to nitrogen can take place. The result issimultaneous reduction of organic carbon, ammonia and total nitrogen.

In another aspect, the invention uses oxygen enrichment as a means ofdealing with peak flows. Need for such oxygen enrichment may bedetermined by on-line COD monitors, or set according to time of day for,for example, municipal applications where diurnal flow and strengthvariations are well known.

In another aspect, the invention uses the module and bioreactor designto conduct other biological reactions on the surface of the fabric. Anexample is biological reduction of compounds such as nitrates in waterusing hydrogen gas supplied to the lumen of the hollow fibre.

In another aspect, the invention uses either air or enriched air tosupply oxygen. Selection of enriched air and level of oxygen present insuch air may be determined by the wastewater strength.

In another aspect, the invention may be used to digest primary and/orsecondary sludge.

In another aspect, fibres, for example fibers of any of the typesdescribed herein, may have a small outside diameter, such as 100 μm orless, and substantial hollow area, for example 30% or more or 40% ormore, so as to have a thin wall. The fibres can be woven, knitted,stitched or otherwise made into a fabric. The use of fine hollow fibresallows the thickness of the fibre wall to be low, for example 20 μm orless, which is several times less than what would be required to make afilm handleable. The fine fibres may themselves be difficult to handleon their own, but may be combined into units such as threads or tows forhandling which may include forming textile sheets. The fabric, having alarge number of hollow fibres, despite the optional use of inexpensivepolymers as the base material, provides sufficient surface area foroxygen transfer capability such that air can be used as a feed gaswithout limiting the growth of the biofilm or other biological kineticsand with acceptable pressure loss due to air flow through the module.

In another aspect, plug flow or multistage continuous stirred or batchtank reactors may be used to conduct biological reactions at the highestpossible substrate concentrations for a given feed. This maximizes masstransfer of organic carbon compounds and ammonia in the biofilm,eliminating these processes as potential limitations to reaction rates.In multi-stage reactors, module designs with lower surface areas foroxygen transfer to biofilm surface area ratios may be used in downstreamstages. The total surface area for oxygen transfer, for example per unitof tank volume or flow rate of feed, may increase or decrease in thedownstream reactor since the lower ratio may result from an increase inbiofilm surface area rather than a decrease in surface area for oxygentransfer.

In another aspect, the invention provides a membrane supported batchbiofilm reactor (MSBBR). The reactor includes one or more membranemodules which are fed an oxygen containing gas and support a biofilmlayer. The modules are located inside of a tank that is cyclicallyfilled and drained to provide a batch treatment process. In anembodiment, the modules are made of a hollow fibre fabric and are usedto reduce the COD, ammonia, total nitrogen and suspended solids in anindustrial wastewater to concentrations suitable for discharge into amunicipal sewer system or for direct discharge to a receiving stream. Inanother embodiment, the modules are used to reduce COD, ammonia, totalnitrogen and suspended solids in a municipal wastewater stream fordirect discharge to a receiving stream. In another embodiment, themodules are used to reduce COD, ammonia, total nitrogen and suspendedsolids in a septic tank to reduce the size of the septic field or to usesimpler, lower cost disposal techniques or for direct discharge to areceiving stream.

In another aspect, the invention provides one or more methods ofcontrolling the growth or thickness of a biofilm layer growing on themodules. Some method(s) involve applying one or more substances to thebiofilm from the tank side while the tank is drained of feed. Thesesubstances may include gases, such as ozone or chlorine, or liquid suchas heated water or basic or acidic solutions. During the application ofthe control substance, conditions in the biofilm may be cycled fromaerobic to anaerobic by turning the supply of oxygen to the inside ofthe module on and off. The biofilm may also be starved prior to theapplication of the control substance by removing the feed water,replacing the feed water with clean water or replacing the feed waterwith feed at a loading of 0.1 kg COD per kg MLSS per day or less. Afterthe application of the control substance, mechanical biofilm controlmethods may also be used on the weakened biofilm.

In another aspect, this invention uses scouring air provided on theoutsides of the fibres as a means of controlling the biofilm thicknessto an optimum level. Air may be used as a means of controlling thebiofilm thickness to a desired level. Treatment with acid, alkali,oxidant, or enzyme, or anaerobic treatment may be used periodicallyprior to air scouring to weaken the biofilm and to improve the efficacyof air in completely or partially removing the biofilm. Other methods ofbiofilm control include in-situ digestion, periodic ozonation followedby digestion, periodic alkali or acid treatment followed by digestion,periodic enzyme treatment followed by digestion, and use of a higherlife form, such as worms, to digest the biofilm periodically. To speedup the biological digestion reactions, the air supplied to inside of themodule may be preheated to raise the temperature of the bioreactor.

In another aspect, the invention provides a tow of hollow fibers, forexample any of the fibers described herein, for example with an outsidediameter (OD) of 500 microns or less or 100 microns or less. Tofacilitate building modules with minimal reduction in the effectivesurface area of the fibres, the fibres are processed or used as towsover a significant portion, for example one half or more, of theirlength. Modules may be made directly from the tows without first makinga fabric. The tows may also be made into open fabrics to facilitatepotting, for example along the edges of the fabric, while leavingsignificant portions of the fibres as tows, for example a portionbetween the edges of the fabric. The modules made from tows may bepotted at both ends, or potted at one end only with the other end leftunpotted with fibre ends open to permit exhaust gas to escape. A singleheader module may have lower cost than a double header module. A singleheader module may be inserted in a vertical configuration with theheader at the bottom and the fibres floating upwards. Such a module maybe aerated from outside the module to remove accumulations of trash andsolids. Feed may also be screened, for example through a 0.5 mm screen,to reduce trash in the feed before it enters the reactor. Where the towmodule is used in a downstream stage of a multi-stage reactor, theupstream stage may also reduce the amount of trash fed to the tow modulereactor.

In another aspect, reactors for treating wastewaters of differentstrength are provided with modules having different ratios of surfacearea for gas transfer to surface area of the attached biofilm. Thesurface area for gas transfer is the area of the outer surface of themodule that is in contact with the supported biofilm. The surface areaof the biofilm is the area of the outer surface of the biofilm thatcontacts the wastewater. Is some cases, the surface area of the biofilmdepends on the thickness of the biofilm which, for calculations or forcomparing modules, may be the actual thickness or time average ofthicknesses of a biofilm in a rector or a nominal or design thickness oraverage thickness, for example 250 microns. A reactor for treatingwastewater with a COD of over 1000 mg/L may have a module with a surfacearea for gas transfer to surface area of attached biofilm ratio of morethan 1, more than 1.6, or between 1.6 and 10. A reactor for treatingwastewater with a COD of less than 1000 mg/L may have a module with asurface area for gas transfer to surface area of attached biofilm ratioof less than 2.5 or between 0.2 and 2.5. A reactor for treatingwastewater with a COD of less than 300 mg/L may have a module with asurface area for gas transfer to surface area of attached biofilm ratioless than 1 or between 0.1 and 10. In a multi-stage process, two or morereactors may be connected in series with the outlet of an upstreamreactor connected to the inlet of a downstream reactor. The COD of thewastewater to be treated decreases through each reactor and the surfacearea for gas transfer to surface area of attached biofilm ratio formodules in a downstream reactor is less than for modules in an upstreamreactor.

In another aspect of the invention, a biofilm is maintained in a stateof endogenous respiration. Endogenous respiration may be achieved bylimiting the F/M ratio of a reactor or limiting the oxygen supply to thebiofilm. In a multistage reactor, an oxygen limited biofilm may be usedupstream of a food limited biofilm. The F/M ratio applied to the oxygenlimited biofilm may be greater than that applied to the downstreambiofilm.

In another aspect of this invention, modules, for example any of thosedescribed herein, are used to supply oxygen to a suspended growthbioreactor. Either pure oxygen, or air, or oxygen enriched air may beused as a source of oxygen in such an application. As oxygen is suppliedin a molecular form to the mixed liquor, it is consumed completely andat very high oxygen uptake rates.

In another aspect of this invention, modules, for example any of thosedescribed herein, are used to supply oxygen to a suspended growthbioreactor, while permitting the growth of a thin biofilm on the fibresurface. The thickness of this biofilm is normally maintained at a verysmall value, for example 100 microns or less due to the lowconcentration of substrate in a suspended growth bioreactor, and due tomixing conditions in the bioreactor, and does not become the controllingfactor in oxygen transfer. Oxygen transfer through the wall of the veryfine fibre may remain the mass transfer controlling step. Provision ofvery high surface area in a module containing very fine fibre, forexample as disclosed herein, permits transfer of large quantity ofoxygen to the bioreactor in a molecular from to permit oxygen uptake ata high rates of 200 mg/h/L of bioreactor volume or greater than 125mg/h/L, or greater than 100 mg/h/L.

In another aspect of this invention, modules, for example any of thosedisclosed herein, are used to supply oxygen to a suspended growthbioreactor, while growing a thin biofilm, for example between 20 and 100microns on the surface of the fibre. Such biofilm is grown to haveimproved nitrification properties due to high oxygen concentration atthe surface of the fibre, and low concentration of biodegradable carboncompounds in the mixed liquor. Under such conditions, growth ofnitrifying organisms is promoted. This promotes partial nitrification inthe biofilm, while supplying oxygen far in excess of that required fornitrification through the fibre wall and the biofilm to the mixed liquorfor organic carbon digestion and nitrification of remaining ammonia inthe suspended media. COD may also be removed in the biofilm in additionto or instead of ammonia.

The features of these various embodiments or aspects may be combinedtogether in various combinations or sub-combinations. Other aspects ofthe invention are described in the claims or in the following drawingsor description.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be described below with reference tothe following figures.

FIG. 1 is a picture of a group of hollow fibres.

FIG. 1 a is a cross-section of a hollow fiber.

FIG. 1 b shows a group of hollow fibers and inert fibers collected intoa unit.

FIGS. 2 a through 2 d and 2 show slot arrangements and a spinneret formelt spinning fibers.

FIGS. 3 a and 3 b show a plan view and cross-section of a woven fabricrespectively.

FIG. 3 c shows steps in weaving a fabric.

FIG. 3 d shows a warp knitted fabric.

FIG. 4 a shows a sheet of hollow fibres with a central portion of thesheet having the fibres in tows. FIG. 4 b shows details of a part of thesheet of FIG. 4 a.

FIG. 5 is a cross-section of a loose tow module.

FIG. 6 shows a top view of a module having sheets of fibres.

FIG. 7 is a partial section, in elevation view, of the module of FIG. 6.

FIG. 8 is a cross-section of another part of the module of FIG. 6 inplan view.

FIG. 9 is an elevation view of a module according to FIGS. 6 and 7.

FIGS. 10 a, 10 b and 10 c are elevation, plan and partial section viewsof another module having sheets of fibres.

FIGS. 11 and 12 are plan and elevation views of a tank having cassettesof modules of sheets of hollow fibres.

FIG. 13 is a drawing of the details of a tensioning mechanism in theapparatus of FIGS. 11 and 12.

FIG. 14 is an elevation view of the mechanism of FIG. 13.

FIGS. 15 and 16 are schematic elevation drawings of reactors.

FIGS. 17 and 18 are schematic drawings of other reactors.

FIG. 19 a is a bench scale batch reactor using a tow module.

FIG. 19 b is a photograph of a biofilm on a tow of fibres growing in thereactor of FIG. 19 a taken through a microscope.

FIG. 20 is a schematic elevation drawing of a septic tank modified touse a supported biofilm module.

FIGS. 21 to 31 are results of tests conducted with various samplemodules or reactors.

FIG. 32 is a schematic illustration of a bioreactor using modulesaccording to the invention to transfer oxygen to a substrate.

DESCRIPTION OF EMBODIMENTS

1.0 Module Elements

1.1 Fiber

FIGS. 1 and 1 a show a Poly (4-methylpentene-1) (PMP) fiber 10 that ishollow inside but non-porous with homogenous dense walls. In a group offibers 10, the fibers 10 may have various diameters, and may be finefibers having outside diameters of less than 500 microns or less than100 microns, for example, between 30 and 100 microns, or between 50 and60 microns. The hollow fibres 10 shown are called non-porous becausewater does not flow through the fiber walls by bulk or advective flow ofliquid water even though there are small openings through the wall,typically in the range of 4 or 5 Angstroms in the embodimentillustrated. However, oxygen or other gases may permeate or travelthrough the fiber walls. The fibers are called dense walled because gastravel is primarily by molecular diffusion or dissolution-diffusionwhich occurs when openings in the fiber walls are generally less than 30Angstroms. The term porous has been used in some publications to referto any structure having openings larger than in a dense wall, forexample having openings of 30 or 40 Angstroms or more, but withoutopenings large enough to be wetted or transport liquid water byadvective, Poiseuille or bulk flow. In this document, membranes orlayers of membranes with openings in this size range will be referred toas semi-porous.

The hollow fiber 10 can be prepared by melt spinning, alternately calledmelt extrusion. In melt spinning a polymer granulate, for example ofPMP, is fed to the hopper of an extruder. The polymer granulate isheated and melted in the extruder and continuously extruded to aspinning head under a pressure of several tens of bars. The spinninghead consists of a heated in-line filter and spinneret. The spinneret isessentially a steel plate with thin arc shaped slots in circulararrangements. Examples of suitable slot arrangements for the formationof a hollow fiber are shown in FIGS. 2 a to 2 d. As shown in FIG. 2, thespinneret may have multiple groups of slots so that many fibers, 8 inthe spinneret shown, can be extruded simultaneously. The molten polymeris extruded through the spinneret, leaves the slots and closes into ahollow fiber in a cooling zone. The gaps caused by the segment dividersallow air into the fibre to prevent collapse before the fibre sectionsfuse to form the annulus. In the cooling zone, the polymer fiber form issolidified and cooled by a controlled cross flow of air and the end iscollected on a take up winder. Suitable fibers 10 may also be formed byother melt spinning methods. For example, in pipe in hole spinning thepolymer is melted and drawn through an annular spinneret while passing agas into the lumen of the extruded fibers through another hole in thespinneret to prevent fiber collapse. Methods other than melt spinningmay also be used. Alternately, the melt spun fibers may be furthertreated after spinning.

Referring to FIG. 1 a, in the illustrated embodiment, fibres 10, forexample fibers of any of the materials or made by any of the processesdescribed in this document, may have an outside diameter 12 of 100 μm orless. The hollow area (or area of the lumen 14) of the fibre may be morethan 10% or more than 30% or 40% of the cross-sectional area of thefibre. The hollow area is typically less than 60% or 50% of thecross-sectional area of the fiber. For example, a polymethyl pentene orPE or PP fibre may be made having an outside diameter 12 of betweenabout 50 to 60 μm and an inside diameter 16 of 30 μm or more, resultingin a wall thickness 18 of 10 μm or less and a gas permeability of over20,000 or 30,000 cc·mm/m²·24 hr·Bar or more.

In the embodiment illustrated in FIG. 1, the textile PMP fibre 10 hasabout a 45 micron outside diameter 12 and about a 15 to 30 micron insidediameter 16. The fibre 10 was melt extruded using MX-001 or MX-002 PMP,produced by Mitsui Petrochemical of Japan and sold under the name TPX,as the raw polymer through a segmented spinneret as described above.This fiber 10 was used in some embodiments and examples described inthis document, although other fibers 10 may also be used.

In another embodiment, a melt spun thermoplastic polymer is mechanicallyor thermally treated after spinning to increase its permeability tooxygen without making the fiber wettable or capable of permittingadvective flow of liquid water. While the precise reason for theincrease in gas permeability remains unclear, it has been proposed thatgas permeability of the fibers 10 is increased by adjusting the spinningprocess parameters to increase crystallinity and enhance molecular chainalignment to obtain a row or stacked lamellar structure. Permeabilityincrease is then obtained by lamella separation of the row lamellarstructure. Another theory is that air permeability can be increased bypreventing the complete growth of stacked lamellar crystals. Either ofthese or other theories may be correct but the inventors do not rely onor limit the invention to any particular theory. Spinning orpost-treatment steps that can be used or controlled to increasepermeability include the spinning speed or drawing ratio, the quenchingconditions such as temperature or air flow rate, post annealing, if any,stretching and heat setting. The resulting fibers may have a denselayer, with openings ranging from the size of openings in the rawpolymer to 30 or 40 Angstroms, on either the inside of the fiber, theoutside of the fiber or both, with the remaining parts of the fiberbeing porous or semi-porous.

Methods of making suitable fibres 10 are known in the art. For example,U.S. Pat. No. 4,664,681, issued on May 12, 1997, to Anazawa et al.describes, in examples 4 and 6, processes for melt-spinning andpost-processing PE and PP to produce acceptable fibres while otherfibers are made from PMP or polyoxymethylene. Processes described in“Melt-spun Asymmetric Poly (4-methyl-1-pentene) Hollow Fibre Membranes”,Journal of Membrane Science, 137 (1997) 55-61, Twarowska-Shmidt et al.,also produce acceptable fibres of PMP and may be adopted to producefibres of other polyolefins such as PE or PP. These references areincorporated herein in full by this reference to them. Although thefibers are non-porous, oxygen, or other gases, may travel through partsof them by one or more of diffusion, dissolution-diffusion or Knudsen'sflow. The fibers are typically asymmetric having a dense layer and asemi-porous layer. Selectivity of the fibers is reduced compared to thebase material. For example, PP has an O₂/N₂ selectivity ratio of over 5but may be 5 or less or 3 or less in a thermo-mechanically treated spunfiber. PMP normally has an O₂/N₂ selectivity of over 3 but may be 3 orless in a thermo-mechanically treated spun fiber. A highly permeablefiber of either, or other, materials may have an O₂/N₂ selectivity of1.5 or less. However, the outer surface of the fibers remainsnon-wetting.

In U.S. Pat. No. 4,664,681, the membranes are melt spun, stretched (byproducing the membrane at a high draft ratio) under weak cooling andthen heat treated. In greater detail, the polymer is melt-extruded at atemperature between T_(m) (the melting point of the polymer) andT_(m)+200° C. at a draft ratio (ratio of final winding speed to actualextrusion speed) or from 20 to 10,000 to create a membrane. Cooling byweak air is provided for between 1 and 30 cm below the spinneret. Theresulting membrane is then stretched amorphously at a temperature offrom T_(g) (the glass transition temperature of the polymers) −20° C. toT_(g)+50° C. and then heat-treated at a temperature of T_(g) toT_(m)−10° C. The resultant membrane is then stretched again at atemperature from T_(g)−50° C. to T_(m)−10° C. and a stretch ratio offrom 1.1 to 5.0. The resultant membrane is then heat-set at atemperature ranging from the last mentioned stretching temperature toT_(m). Other or additional steps were also used in various embodiments.The resulting membranes had increased oxygen permeabilities. O₂/N₂selectivity was reduced but remained above 1.2. The fibers areasymmetric containing a dense layer with substantially no pore with adiameter of 30 Angstroms or more and a microporous layer having largerpores. The non-porous layer is the outer surface of the fiber and so thefiber is non-wetting.

In the paper by Twarowska-Shmidt et al., PMP fibers are melt spun with avariety of cooling means below the spinneret, most notably a screen,made of a cylindrical blower not producing air, installed below thespinneret to restrict cooling of the unsolidified fibers as they exitthe spinneret. Melt temperatures ranged from about 277 to 310° C., takeup speeds ranged from 500-1,000 m/min and extrusion rates, through aspinneret with 8 holes of 1 mm diameter and 0.1 mm slot width or 2 mmdiameter and 0.15 mm slot width, was 5.6-13.3 g/min. After spinning, thefibers were annealed (in a package at 180° C. in at heat chamber for 300to 3000 seconds or by passing a thread at a rate of 20-60 m/min througha 2 m long heated chamber), followed by one step stretching at roomtemperature at a draw ratio of 1.15, followed by relaxation to 7%,followed by heat setting at 180° C., followed by stretching at a rate of265 m/min. The resulting membranes had increased oxygen permeability andreduced O₂/N₂ selectivity but, for at least those with the screen belowthe spinneret, O₂/N₂ selectivity remained about 1.7 indicating anasymmetric structure. Mean pore size of the fibers produced using thescreen was just over 40 Angstroms.

Suitable fibres 10 may also be formed by methods other than meltspinning. Such other methods may include, for example, but not limitedto, meltblown extrusion, flash spinning, and electrospinning. Inmeltblown extrusion, fibres are formed by extruding molten polymerthrough spinneret orifices. As the filaments exit from the orifices,they are attenuated by high temperature, high velocity air streamsbefore being deposited into a conveyor belt to dry.

In flash spinning, pure solvent droplets and highly saturatedpolymer/solvent mixtures are decompressed through a spin orifice. As thepressurized solution is allowed to expand rapidly through the orifice,the solvent is “flashed off” instantaneously leaving behind athree-dimensional film-fibril network.

Electrospinning uses an electric field to draw a polymer melt or polymersolution from the tip of a capillary to a collector. A voltage isapplied to the polymer, which causes a jet of the solution to be drawntoward a grounded collector. The fine jets dry to form polymeric fibres,which can be collected on a web. By choosing a suitable polymer andsolvent system, fibre diameters can be varied and controlled.

1.2 Fiber Aggregates (e.g. Tows)

Referring to FIG. 1 b, the hollow fibers 10 may be combined into fiberunits 19 for handling. The fibre units 19 may be individual fibres 10,tows 20, for example, of 1 to 200 or 16 to 96 fibres 10 each, eithertwisted or untwisted (FIG. 1 b), threads, yarns, tubular, flat orcordage braids, or other units 19 for handling. Tows 20 are made byre-winding fibers from multiple take up spools in combination on to asecond spool. Stronger inert fibres 22, such as yarns of unstretched PEor PP, may be included in a tow 20 or other unit 19. The fibres 10 maybe curled for use in the units 19. Curled fibres 10 can be made bywinding them onto a bobbin at varying tensions.

1.3 Sheet Structures

The fibers 10 and/or fiber units 19 can be provided in the form ofsheets 26. In FIGS. 3 a and 3 b, the fibres 10 are woven as fibre units19 into a basic two-dimensional structure or fabric sheet 26. In theembodiment illustrated, the units 19 run across the sheet, meaningperpendicular to the direction in which the sheet 26 advances out of aloom. Inert fibres 22 run along the length of the sheet 26 to providesupport to the fibre units 19. FIG. 3 c illustrates steps involved in aweaving process. The fiber units 19 are carried on a shuttle through 2groups of inert fibers 22 that are alternately raised or lowered aftereach pass of the shuttle. Other weaving or fabric making methods mayalso be used. Unit 19 type, unit 19 bundle size, spacing between units19 and percent of fibre in each direction can all be tailored to meetthe mechanical or biochemical requirements of each unique application.Further, surface roughness of the sheet 26 can be controlled to aid inbiofilm control. For example, it may be easier to control the biofilm ona sheet 26 with a rough or textured surface, for example, one in whichthe height of the surface undulations roughly matches the desired rangeof biofilm thickness, which may be, for example, from 200 to 1,000microns.

In more detail, the fibre units 19 provide a support surface for thegrowth of a biofilm 30. The number of hollow fibre units 19, and thenumber of fibres 10 per unit 19, may be adjusted to provide a desiredsurface area for O₂ transfer compared to surface area of biofilm 30 orto the planar surface area of the fabric sheet 26. The planar surfacearea of the sheet 26 is simply the sheet length multiplied by its width,multiplied by two (since the sheet has two sides). The surface area ofthe biofilm 30 is the total area of the biofilm 30 exposed to the liquidin the reactor, which may be generally the same as the planar area ofthe sheet 26 for a substantially two dimensional sheet configuration.

The surface area for O₂ transfer is the total area of the hollow fibres10 in the sheet exposed to the biofilm. This is approximately equal tothe product of the effective diameter and length of the fibre 10,multiplied by the number of the fibres 10 in the sheet 26. The effectivediameter for diffusion is a logarithmic average of the diameters of thefibre to account for the effect of the wall thickness. The inert fibres22 crossing the hollow fibres 10 in the sheet 26, and contact betweenfibres 10, may interfere with oxygen transfer in some embodiments, forexample a tightly woven fabric, but the interference is generally smalland is ignored in surface area for oxygen transfer calculations.

Although the surface area of the biofilm 30 is generally the same as theplanar area of the sheet, it may be slightly larger for very rough oropen fabrics or fabrics having more dispersed fiber units 19. Varyingfabric roughnesses may also be used to affect the thickness of thebiofilm 30 or how readily the biofilm 30 can be reduced or controlled.High ratios of O₂ transfer surface area to biofilm area (SA O₂/SAbiofilm) may be obtained, in the range of, for example, 6 to 10 or more.However, for treating feed water with a high concentration of COD, forexample, 300 mg/L CODs or more, lower SA O₂/SA biofilm ratios, forexample, between 1.6 and 10 are sufficient, and may be preferred toreduce module cost. An SA O₂/SA biofilm ratio in the range of about 2 to8, or about 4 to 6, can provide satisfactory results in many treatmentapplications.

The surface area of the biofilm 30 can also be larger than the planararea of the sheet 26 by providing a loose arrangement of fibres 10 andcontrolling the thickness of the biofilm 30 to a sufficiently thin layerso that the biofilm 30 on adjacent parallel fibres does not form acontinuous layer. A sheet 26 with a rough or textured surface, theheight of the surface undulations being in the range of the desiredbiofilm thickness, may also be desirable since it may facilitate biofilmcontrol. Desired biofilm thickness may be 200 to 1,000 microns.

Provided that oxygen transfer through the module 40 does not limitreactions in the biofilm 30, the rate of COD reduction in the wastewateris roughly proportional to the concentration of COD in the wastewater.However, for oxygen transfer to not be a limiting factor, more oxygen isrequired to flow through the module 40 to support a biomass of the samesurface area as wastewater COD concentrations increase. More oxygen canbe provided by increasing the size or speed of operation of a blower.However, large head losses, for example 10 psi or more, may result dueto resistance to oxygen flow through the fibre lumens 14. Head loss maybe kept below 10 psi, or in the range of 6 to 9 psi, by choosing afabric type and number of fibres that produces sufficient total lumenarea for a given biofilm outer surface area.

Also, the inventors have observed that biofilms growing in wastewaterwith high concentrations of COD, for example 1000 mg/L CODs or more or2000 mg/L CODs or more, are more resilient and tend to grow toundesirable thickness of a few mm or more, faster than biofilms growingin wastewater with lower COD concentrations. Thus, biofilms growing inhigh COD wastewater require more strenuous biofilm control methods whichin turn make a stronger fabric desirable.

The various issues discussed above make it preferable for fabrics to beused in high COD wastewater that have more fibres, and optionally moresurface roughness, for the same overall planar area of a sheet or outersurface area of supported biofilm than for fabrics used to treat lowerCOD wastewater. This can be achieved by choice of method used to createthe fabric and choice of thread or fabric unit count or tightness of thefabric. Multi-stage reactors may also be used. In a multi-stage reactor,an upstream reactor treats the feed at its highest COD concentration andis fitted with modules having dense fabrics with large numbers offibres. A downstream reactor receives partially treated wastewater witha lower COD and is fitted with modules having a less dense fabric withfewer fibres for the same sheet or biofilm outer surface area. The lessdense fabric is more economical since it has less fibres and may have ahigher area of biofilm for a sheet of the same planar surface area.

The fabric sheets 26 may also be made by other methods such as braiding,stitching or knitting, such as warp knitting. Warp knitting isdesirable, for example, when small units 19 or tows or even individualstrands of fine fiber 10 are used. The fabric sheets 26 may bepatterned, as in pattern knitting, if desired, to provide areas withfewer fibers or holes to enhance flow through the sheets 26.

In warp knitting, the fabric sheet 26, as shown in FIG. 3 d, containsinterlaced loops of ‘knitted stitches’. The column of stitches beingformed on one needle make a fringe. The fringes in the length direction(‘warp’) of the fabric can be made by relatively inexpensive, commodityyarns, e.g. PET, PP, etc., as the inert fibres 22. The inert fibres 22can withstand the stress and strain of processing and use. The fabricsheet 26 is generally strong and stiff in the warp (length) directionand elastic in the weft (cross) direction. The weft is a perpendicularyarn system, which is laid across the fringes and fixed by stitches(loops) of the warp fibres 22. The weft doesn't take part in the fabric(loop) formation, therefore the weft fibre units 19 can be processedvery tenderly, being subjected to less stress and strain than the warp.Accordingly, preparing the sheet 26 with units 19 as the weft canminimize risk of damage to the fibres 10 during manufacturing the sheet26. The weft is usually a parallel layer or band of yarns being movedcrosswise to the fringes (warp) during knitting. The fabric sheet widthcan be about to 2-3 m.

In the embodiment of FIGS. 4 a and 4 b, the sheets 26 are constructed ofan open fabric made by weaving tows 20 through the shuttle of a loom andcrossing the tows 20 with an inert fibre 22 only along the edges of thefabric 26. The fabric shown is approximately 1.3 m wide, that is it hasactive fibres 10 of about 1.3 m long, and has inert fibers 22 wovenperpendicularly to the tows 20 in a strip of about 2 cm along the edges.As shown in FIG. 4 b, the fibers 10 in each tow 20 disperse beyond thestrips so that the tows 20 remain unrestrained and partially openbetween the strips. The resulting roll of 1.3 m wide fabric is cut intosections of about 20-200 cm, or 30-60 cm, width to make individualsheets 26. In FIG. 4 b, the number of fibers 10 in each tow 20 is smallfor clarity but the tows 20 may each have, for example, between 1 and200, for example 16, 48 or 96 fibers 10.

1.4 Modules

1.4.1 Loose Tow Module

In accordance with the present invention, multiple fiber units 19,including fibers 10, tows 20 or sheets 26, can be grouped together toform membrane modules 40. FIG. 5 shows a module 40, which may be calleda tow or loose tow module, with fibres 10 arranged and potted in tows 20of fibres. The tows 20 are made of a loose collection of a plurality offibres 10, for example between 1 and 200 or 16 to 96 fibres 10. Thefibres 10 may be lightly twisted together or left untwisted. The fibres10 may be curled, crimped or undulating to provide three dimensionalstructure to the each potted row. Curling may be achieved by re-windingthe fibres 10 onto a bobbin while varying the tension on the fibres. Theindividual fibres 10 remain separable from each other in the tow 20.Such a tow 20, when coated with a thin biofilm, for example of less than1 mm thickness, may provide a ratio of gas transfer area through thefibre walls to biofilm outer surface area (SA_(oxygen)/SA_(biofilm)) ofless than 2.5, less than 1 or between 0.1 or 0.2 and 1. Inert fibres 22may be added to the tow to strengthen it if required. Each tow 20 ispotted into a plug of resin 32 so that its ends 34 are open at one faceof the resin 32. The plug of resin 32 is glued into a plastic headerenclosure 35 having a port 36 which forms a header 44 connecting theport 36 to the open ends 34 of the fibers 10 through a cavity 37. Thereare two headers 44, one associated with each end of the fibres 10,although modules 40 with only an inlet header 44 may also be made. Withtwo headers 44, air or other gases may be input into one header 44, flowthrough the fibres 10 and exhaust from the second header 44. Tows arepotted in a resin 32, such as polyurethane, and the potted ends are cutto expose the fibre lumen. Alternately, a fugitive potting material maybe used to block off fibre ends, as described in U.S. Pat. No.6,592,759, or other potting methods may be used. In FIG. 5, the numberof tows 20 and the number of fibers 10 per tow 20 are both small forclarity in the drawing and may be much larger in practice.

1.4.2 Sheet Module

A module 40 can also be constructed of a bundle or stack of sheets 26.The sheets 26 may have perpendicular inert fibers present across theentire width of sheet 26 as in FIG. 3 a or only across a portion of thewidth of the sheet 26, for example at the ends as in FIG. 4. Rawmaterial for the sheets 26 may be rolled onto a fabric roll. Forexample, where the sheets 26 are prepared by weaving, the material isrolled on to a take up roll at the end of a loom as material isproduced. The fiber units 19 may extend across the roll while the inertfibers spiral around the roll. With the fibers oriented in this way,individual sheets 26 may be cut from the roll by rolling out a length ofmaterial from the roll and cutting it off with a hot knife or heatcutter. The heat cutter melts through the fiber units 19 and inertfibers and bonds them together to protect the fabric edge fromdisintegrating or fraying. Since the heat cutter melts a strip of fiberson either side of the cut line, for example a strip about 5 mm wide, thefibers remaining on the roll are similarly melted together to produce astable edge. After a sheet 26 has been cut from the roll, the other twoends of the sheet, meaning the edges of the sheets 26 at right angles tothe heat cut edges, are cut to open the lumens of the fiber units 19. Tominimize distortion or collapse of the ends of the fibers 10 under thecutting pressure, the area to be cut is first reinforced, for example byimpregrating it with polyurethane to provide a reinforcing coatingaround the fibers 10 or fiber units. The cut across the fiber units 19is then made with a sharp cutter, for example a razor edge cutter. Thecutter is preferably kept very sharp, for example by changing bladesregularly, to minimize distortions of the ends of the fibers 10. Othercutting machines or tools used in the garment and textile industries mayalso be used.

The end or ends of single or multiple sheets 26 can be potted into aheader to provide one or more ports 36 in communication with the lumensof the fibers 10. To pot one or more sheets 26, sheets 26 are cut from aroll as described above. A plastic spacer strip is attached, for examplewith glue or adhesive transfer tape, on one or both sides of the sheet26, at the end of the sheet 26 parallel to but offset from the razor cutline across the fiber units 19. For potting multiple sheets 26, thesheets 26 with spacer strips attached are laid on top of each other andattached together, for example by glue or adhesive transfer tape,between adjacent spacing strips or between the spacing strip of onesheet 26 and a second sheet 26. The strips space adjacent sheets 26 butalso form a barrier between a potting material to be applied later andthe cavity of the header containing the ends of the fibers 10. The endsof the sheet 26 or stack of sheets 26 is fitted into an elongated headercavity that may be made, for example, by injection molding. Spacing andsealing to the header walls is maintained with a self-adhesive closedcell neoprene gasket strip attached to each of the long header walls.Any openings in the header cavity left by the spacer strips may becovered with hot melt glue. Final sealing of the header is achieved bypouring a layer of potting material, for example a two-componentpolyurethane compound, over the spacer strips. The layer may be about 45mm thick and extend between the insides of walls of the header. If thereare multiple sheets, care is taken to force or ensure flow of thepotting material, as completely and evenly as practicable, between thesheets 26. After the potting material hardens, a seal is formed betweenthe outsides of the fibers 10 and the walls of the header but the endsof the fibers 10 remain in communication with a cavity within theheader.

FIGS. 6 to 9 show a module 40 in which a set of parallel sheets 26 arepotted with gaps 42 between them in a header 44. Two headers 44 may beused as shown when a bleed of exhaust air is desired. One header 44 mayalso be used with exhaust bled through opposed open ends of the fibres10 or with the other ends of the fibres 10 closed for dead endoperation. The gap 42 may be between 2 mm and 10 mm thick, or between 3mm and 15 mm. The chosen gap 42 may depend on the water to be treated orthe choice of method to control biofilm thickness. For example, a module40 of tensioned sheets 26 may have a gap 42 of 6 mm when used with airscouring to control biofilm 30 thickness. Tension may be provided bymounting the headers 44 to a rigid structure, which may include parts ofa tank, with one or both headers 44 movable relative to the structure.Alternately, the headers 44 may be attached to part of a frame held atan adjustable distance apart. The sheets of fabric 26 are potted andseparated in the headers 44 by various potting materials 46 such as oneor more of polyurethane, hot melt glue, adhesive strips, plastic spacingstrips or epoxy. The spacing between adjacent sheets 26, or gaps 42,provides space for scouring air and substrate flow through the module40. A large sheet of the fabric 26 may also be rolled or folded toproduce a module 40 rather than using separate sheets. The length of themodule 40 is a compromise between OTE and pressure drop and may rangefrom 1 m to 5 m or between 1 m and 3 m.

Referring to FIG. 8, to make the module 40 a sheet 26 of fibres 10 islaid onto strips 50 (one on each end) of adhesive located to cross theends of the fibres 10. Additional strips 50 of adhesive and spacingstrips 52 are placed over the sheet 26, followed by additional strips 50of adhesive and an additional sheet of fabric 26. These steps arerepeated as appropriate for the number of sheets 26 desired. Theresulting assembly is then sealed into the header enclosures 35 of apair of opposed headers 44 such that the lumens 14 of the fibres 10 arein communication with ports 36 in the headers 44 through cavities 37.The ends of the fibres 10 are cut before potting to open them, forexample as described above. Additional glue or potting resin 41 mayoptionally be poured into the header enclosure 35 to further seal thefibers 10 to the header enclosure 35. Alternately, sheets 26 may beseparately glued to spacing strips at their edges and inserted into aheader cavity and additional glue or potting resin 41 placed around thisassembly to seal it to the header enclosure 35. Further alternately, thefirst assembly method described above may be used.

FIG. 9 shows a picture of a module 40 assembled as generally describedabove. The headers 44 are about 2 meters apart. Additional spacers 33are used mid way between the headers to better preserve the sheet 26separation. A thin steel rod 45 is attached to the edges of the fabricsheet 26 in the right half of the module to address the folding whichcan be seen in the left half of the module. The module 40 has a ratio ofSA oxygen/SA biofilm of about 5.

Another embodiment of a module 40 can be seen in FIGS. 10 a to 10 c. Themodule 40 has a single sheet 26 with hollow fibre units 19 and inertfibres 22. The hollow fibre units 19 extend between headers 44 at eitherend of the sheet 26. The width 62 of the headers 44 is such thatstacking multiple modules 40 adjacent each other with the headers 44 ofadjacent modules 40 abutting each other provides the desired spacingbetween the adjacent sheets 26. The header enclosures 35 of this module40 are clear allowing the cavity 37 to be seen. To pot the sheet 26, theheader enclosure 35, which is a folded over plastic strip, is forcedopen and a sheet 26 is inserted. The header enclosure 35 springs closedon the sheet 26. Tubes which function as ports 36 are inserted into theends of the header enclosures. Potting resin 31 is laid along the jointbetween the sheet 26 and the header enclosure 35, between the ports 36and the header enclosure 35 and all other openings to seal the cavity37.

Referring again to FIG. 4, another module, which may be called a tow ortow sheet module, can be made of open sheets 26 of tows 20 cut along thewoven edges to open the ends of the fibres 10 and potted with a 0 to 10mm space between them into one or a pair of opposed headers. Dependingon the potting method used, which may include potting methods describedabove, the fibres 10 may be cut open either before or after they areinserted into the potting resin. 1 to 100 or 8-20 sheets may be pottedinto a pair of headers to produce a module. Modules made in this wayusing the fibers of FIG. 1 had SA_(oxygen)/SA_(biofilm) ratios ofbetween 1:2.5 (0.4) and 1/11 (0.1) with a biofilm thickness of 250microns.

1.5 Cassettes/Reactors

In general, a plurality of modules can be grouped together to form acassette, and one or more modules or one or more cassettes can be placedin a tank as part of a reactor. Referring to FIGS. 11 and 12, themodules 40 of a cassette 110 are mounted in a tank 112 of a pilotreactor for treating 1 cubic meter per day of industrial wastewaterhaving a COD of over 1,000 mg/L, typically 7,000 mg/L. The feed istreated by either batch or continuous process to reduce its CODconcentration to 300 mg/L as required for discharge into the municipalsewer that it outlets to. The tank 112 has a fill volume of 1.8 m³.Fifteen modules 40 are provided in the tank 112, each module 114containing six sheets 26 of 3.6 m² surface area of a woven fabric of PMPfibers units 19, woven as tows 20. The fibres 10 are 1.8 m long andextend between an inlet header 116 and outlet header 122 of the modules40. Total number of PMP tows per sheet is 1968, and fibres per sheet are94464, there being 48 fibers per tow and a two packing of 50 threads perinch in the sheet 26. Also, polyester yarn is woven perpendicular to thePMP fibre, and the total number of yarns per module is 1912. Airpressure drop in the fibre lumen is in the range of 5 to 10 psi. Totalbiofilm area per module is 17 m², and oxygen transfer area is about 5.1times the biofilm area.

The modules in the embodiment illustrated are mounted in such a way thatthe tension of the sheets 26 extending between the headers 116, 122 canbe adjusted. The cassette provides a rigid structure 150, which caninclude elements of the tank 112 or elements of a cassette sub-frame,adjacent the modules 40, and one or both of the headers 116, 122 aremovable relative to the rigid structure 150.

In the embodiment illustrated, the rigid structure 150 comprises a pairof side plates 152 that extend along the distal side surfaces of theoutermost modules 40 of the stack of modules 40. As best seen in FIGS.13 and 14, the modules 40 are attached to the side plate 152 by means ofa mounting bracket 154 extending transversely between the side plates152 at either end of the modules 40. The mounting brackets 154 areprovided with grooves 156 shaped to receive T-shaped tongues 158extending from surfaces of the headers 116, 122, opposite the sheets 26.The module 40 can be secured to the mounting brackets 154 by sliding thetongues 158 of the headers 116 and 122 into the grooves 156 of thebrackets 154. The mounting brackets 154 can be secured to the side plate152 by, for example, a bolt 160 passing through an aperture 162 engagingthe plate 152 and a threaded hole 164 in an edge surface of the bracket154.

The aperture 162 can be slot-shaped, so that the bracket 154 with theattached header 116, 122 can be shifted horizontally to increase ordecrease the tension of the sheets 26. An eccentrically mounted cammember 166 can be provided between the head of the bolt 160 and theplate 152, with an outer diameter surface in engagement with an abutmentsurface 168 fixed to the plate 152. Rotating the cam member 166 canforce the opposed brackets 154 further apart or allow them to drawcloser together, thereby adjusting the tension of the sheets 26 in themodules 40.

The tension adjustment mechanism can be provided on only one end or onboth ends of the modules 40, and can be modified to provide individualtension adjustment for each module 40 or for sub-groups of modules 40.Other mounting methods may also be used to allow modules 40 to beremoved or tensioned.

In another embodiment of the invention, the elements or modules arestacked in a vertical configuration. Flow of scouring air from outsidethe modules or of water in the tank may be from top to bottom or bottomto top. This minimizes the capital required for scouring air and theoperating cost of air.

2.0 Operation/Applications

The fiber units 19 having one or more fibers 10 can be used as membranesto support biofilm in a reactor. In general, gas containing oxygen flowsinto at least one of the headers 44 of a module 40. The module 40 may beoperated in a dead end mode, with no outlet other than through thefibres. Alternately, the module may be operated in a cross flow mannerwith gas entering through one header 44, flowing through the fibers 10,then exiting from the other header 44. The oxygen content and flow rateof the gas may be set to produce an oxygen transfer that providesaerobic conditions near the outer surface of the fibers 10, where thelevel of oxygen is highest. Aerobic reactions occur in this area,including conversion of organic compounds to carbon dioxide and water,and ammonia to nitrates. The biofilm may be maintained under anoxicconditions on its outer surface or near the substrate being treated andconversion of nitrogen to nitrates can take place. In this way, multipleand simultaneous reactions, including carbon based organics, ammonia andtotal nitrogen reduction, may be performed in the biofilm.

Air may be used as an oxygen bearing gas input to the modules 40. Eventhough N₂ or CO₂ selectivity may be low, the partial pressure gradientbetween N₂ or CO₂ on the inside of the fibres and the biofilm orsurrounding substrate is not large. Accordingly, substantial amounts ofN₂ or CO₂ do not diffuse into the substrate. In contrast, CO₂ mayback-diffuse into the fibre lumens 14 particularly with a lowselectivity membrane. The exhaust gas, collected at the exhaust header,may be bubbled through a lime slurry to recover the CO₂ and preventdischarge of CO₂. The flow rate of CO₂ enhanced gas to be treated ismuch smaller than for a conventional wastewater treatment processallowing for more efficient control of CO₂ emissions.

An example reactor 80 is shown in FIG. 15. FIG. 15 provides a near plugflow. The reactor 80 has a tank 82, a feed inlet 84 to the tank 82, aneffluent outlet 86 from the tank 82, a flow path 88 between the feedinlet 84 and the effluent outlet 86, and a plurality of fiber units 19in the form of modules 40 in the tank 82. Each module 40 can have one ormore sheets 26 extending from one or more headers 44. The plurality ofmodules 40 can be provided as part of one or more cassettes 110.

The sheets 26 and modules 40 are sized to fit the tank 82 and fill asubstantial part of its volume. The sheets 26 may be custom made toprovide efficient use of the available space in the tank 82. The sheets26 are preferably arranged in the tank 82 in a number of rows, one suchrow being shown in FIG. 15. The sheets 26 may range from 0.25 to 2 mm inthickness and adjacent sheets 26 are placed in the tank 82 side by sideat a distance of 2 to 15 mm to allow for biofilm growth and wastewaterflow between adjacent sheets 26.

The tank 82 is longer than it is deep and may have a generallyhorizontal flow path 88 with minimal mixing. This is achieved by leavingsome space near the ends (ie. near the inlet 84 and outlet 86) of thetank 82 for vertical movement of water and leaving minimal free space atthe top, bottom and sides of the tank 82. A baffle 90 may also be placedupstream of the effluent outlet 86 to force the flow path 88 to go underit. A sludge outlet 92 is provided to remove excess sludge.

The flow path 88 is generally straight over a substantial portion of thetank 82 between the feed inlet 84 and effluent outlet 86. Each module 40is held in the tank 82 by its headers 44 attached to a frame (not shownfor clarity) which restrains each module 40 in positions in the reactor80 whereby the sheets 26 of each module 40 are generally parallel to theflow path 88. Preferably, a plurality of sheets 26 are spaced in seriesalong the flow path 88 so that the reactor 80 will more nearly have plugflow characteristics. Wastewater to be treated may be partially recycledfrom the effluent outlet 86 to the feed inlet 84. Such a recycle canincrease the rate of gas transfer by increasing the velocity ofwastewater along the flow path 88, but it is preferred if the recycleratio is small so as to not provide more nearly mixed flowcharacteristics in the reactor 80.

Oxygen containing gas is provided to each module 40 through its inletconduit 216 connected to an inlet manifold 94 located above the water tobe treated. With the inlet manifold 94 located above the water, a leakin any module 40 will not admit water into the manifold nor any othermodule 40. Gas leaves each module 40 through its outlet conduit 218which is connected to an exhaust manifold 95. Although it is notstrictly necessary to collect the gases leaving each module 40, it doesprovide some advantages. For example, the gas in the exhaust manifold 95may have become rich in volatile organic compounds which may createodour or health problems within a building containing the reactor 80.These gases are preferably treated further or at least vented outside ofthe building.

Oxygen diffuses or permeates through the fibers 10. The amount of oxygenso diffused or permeated may be such that an aerobic biofilm is culturedadjacent the sheets 26, an anoxic biofilm is cultivated adjacent theaerobic biofilm and the wastewater to be treated is maintained in ananaerobic state. Such a biofilm provides for simultaneous nitrificationand denitrification. A source of agitation 98 is operated from time totime to agitate the sheets 26 to release accumulated biofilm. A suitablesource of agitation is a series of coarse bubble aerators which do notprovide sufficient oxygen to the water to be treated to make itnon-anaerobic.

FIG. 16 shows a second reactor 80 having a tank 82, a feed inlet 84, aneffluent outlet 86, a flow path 88 and a plurality of modules 40. Frames(not shown) hold each module 40 in a position whereby the sheets 26 ofeach module 40 are generally parallel to the flow path 88.

The sheets 26 are sized to fit the tank 82 and fill a substantial amountof its volume. The sheets 26 may be custom made to provide efficient useof the available space in the tank 182. The sheets 26 may range from0.25 to 2 mm in thickness and are placed side by side at a distance of 2to 15 mm to allow for biofilm growth and wastewater flow betweenadjacent sheets 26.

The tank 82 is deeper than it is long to encourage a straight andgenerally vertical flow path 88 over a substantial portion of the tank82 with minimal mixing. This is done by leaving minimal space near theends and sides of the tank 82 but a substantial amount of space near thetop and bottom of the tank 82. Water to be treated may be partiallyrecycled from the effluent outlet 86 to the feed inlet 84 but it ispreferred that the recycle rate be small if a recycle is used.

Oxygen containing gas is provided to each module 40 through its inletconduit 216 connected to a manifold 94. The manifold 94 may alternatelybe located above the water to be treated so that a leak in any module 40will not admit water into the manifold 94 nor any other module 40.Outlet conduits 218 are connected to an outlet manifold 95 which mayalternately be located above the surface of the water to be treated.

Alternatively, gas flow through the module 40 is produced by applying asuction to the outlet conduits 218. The inlet conduits 216 are placed influid communication with the atmosphere. By this method, the rate of gasdiffusion across the membrane is slightly reduced, but the exhaust fromthe blower may be connected to further apparatus for processing theexhaust gases.

Oxygen diffuses or permeates through the membranes 120 preferably suchthat an aerobic biofilm is cultured adjacent the sheets 26, an anoxicbiofilm is cultivated adjacent the aerobic biofilm and the wastewater tobe treated is maintained in an anaerobic state. A source of agitation 98is operated from time to time to agitate the sheets 26 to releaseaccumulated biofilm. A suitable source of agitation is a series ofmechanical mixers.

Referring to FIG. 17, a reactor 100 has a tank 112 with one or moremembrane supported biofilm module cassettes 110 installed inside of it.The cassettes may have one or more modules 40, as described above. Themodule 40 may also be a tow module, a module of planar elements, orother types of modules using a membrane to support a biofilm. Eachmodule 40 has a gas inlet header 116 fed with air, or another oxygencontaining gas, through a blower 118. Gas passes from the inlet header116 to the inside (or lumens 14) of one or more fibers 10. The walls ofthe fibers 10 serve as gas transfer membranes 120. A portion of the gaspasses through the membranes 120 while another portion, and possiblysome gasses taken up from the tank 112, flow to an outlet header 122 ofthe modules 40 and to an exhaust 124. The gases leaving the exhaust 124may be post-treated or discharged to the atmosphere.

Feed water enters the reactor 100 through a feed valve 126 and feed pump128. The feed is filled to a feed fill level 130 above the modules 40.After a batch of feed has been treated, a drain valve 131 is opened todrain the tank 112 of treated water. The treated water may flow to amunicipal sewer, to the environment, discharge directly to a receivingstream, or to another stage of a MSBBR (membrane supported biofilm batchreactor) or to another sort of reactor for further processing.

A biofilm 132 grows on the outside of the membranes 120. To control thethickness of the biofilm 132, one or more aerators 134 are providedbelow the modules 140 and connected to a scouring air blower 136 throughan aeration valve 138. The scouring air blower 136 can be operated toprovide bubbles when the tank 112 is full of water. The bubbles risethrough the module 140 and physically remove some of the biofilm 132from the membranes 120. The aerators 134 are also attached to a gassupply 140 through a gas supply valve 142. The gas supply 140 maycontain a pressurized gas or a gas generator and pump or other devicefor supplying a gas when the tank 112 is empty. The reactor 100 also hasa liquid pump 144 operable to fill the tank 112 with a liquid other thanfeed water. The liquid pump 144 may be connected to a reservoir holdingthe liquid or to a source of clean water passing through a modifier,such as a chemical injection device or heater. The tank 112 is generallyopen to the atmosphere and contains liquid at generally ambient pressurebut has a lid 146 which may be closed from time to time to provide anenclosed space.

The main treatment process in the reactor 100 involves the batchapplication of feed to the biofilm 132. The tank 112 is filled with feedto the feed level 130 using the feed pump 128. The feed pump 128 isconnected to the feed supply through an equalization reservoir 148 topermit batch operation from a non-batch feed. The feed remains in thetank 112 for a period of time, for example between 12 and 96 hours,while it is treated by the biofilm 32. During treatment, the lid 46 mayremain open, but the water in the tank 112 is generally anoxic oranaerobic. However, oxygen, typically as a component of air, is suppliedto the biofilm 132 through the membrane 120 by the blower 118 creatingan aerobic region on the biofilm 132. From time to time during thetreatment period, a recirculaton valve 149 may be opened and feed pump128 operated to mix the feed water in the tank 112.

After the biofilm 132 has digested the feed to the desired degree, thedrain valve 131 is opened to drain the tank 112. The draining may occurin two steps. In the first step, the solids slurry present in the bottomof the tank is drained to remove settled solids which are thentransferred to a sludge management system. In the second step, the cleardecanted liquid is then drained to a second stage treatment ordisinfection system, or discharged to a sewer, or discharged to areceiving stream.

The oxygen bearing gas supply may be continued throughout the fillingoperations to continue digestion of the material adsorbed on thebiofilm, and to ensure that treatment starts immediately as soon as aportion of the biofilm is immersed in the wastewater. Similarly aerationmay continue throughout the draining operation to continue treatment aslong as a portion of the biofilm is immersed and to digest organicsadsorbed in the biofilm for a short period of time even while notimmersed, so as to maximize the time of treatment of each batch.

Referring now to FIG. 18, a reactor 400 is shown having similar featuresas the reactor 100, but without the gas supply 140, gas supply valve142, or liquid pump 144.

In a batch process, the concentration of the wastewater decreasestowards the end of each processing period. Demand for oxygen supplied tothe biofilm also decreases and so the gas supply to the modules may bereduced. Modules using fibres at least partially in the form of towsallow a very high surface area for oxygen transfer and biofilm growth.Tow modules are particularly useful in treating wastewater having a lowCOD, for example 1,000 mg/L or less, 500 mg/L or less or 300 mg/L orless, because they provide large surface areas. Pressure loss throughthe fine fibre lumens is not limiting with the amount of air supplyrequired to deliver oxygen to a biofilm treating low COD wastewater.Although they may be useful for treating other wastewaters as well, towmodules can be used where the initial feed has a low COD or as a secondor third stage behind other treatment processes or apparatus that reducethe COD concentration of stronger feedwaters. With municipal wastewateror other feeds, for example feeds having a COD of 1,000 mg/L or more, atwo stage apparatus may be used. In a first stage, membrane supportedbiofilm modules in the form of a fabric sheet are used as in FIG. 9. Theoutlet from a reactor containing these modules is fed to a reactorcontaining tow modules with sheets as in FIG. 4 which provides secondstage treatment. The inventors have observed that rapid reduction in CODfrom a high COD wastewater limits the denitrification produced from amembrane supported biofilm reactor. With a two stage process, the firststage may be optimized for COD removal. The feed to the second stage hasa reduced COD and the second stage may be optimized to supportnitrifying microorganisms, for example of the species nitrobacter andnitrosomas, over carbon degrading microorganisms to provide improvedammonia oxidation in the second stage.

In general, when considering COD, soluble COD is used since soluble CODis most easily digested by a biofilm 30 and is easily measured. However,particularly for modules 40 with loose tows 20 over some or all of theirarea, some particles of insoluble COD are trapped in the biofilm. Overtime, these particles are broken down into soluble COD and digested.Accordingly, total, or total biodegradable, COD also may be a relevantparameter in some embodiments.

For feeds having a CODs of 1000 mg/L or more, a module 40 may have anSA_(OXYGEN)/SA_(BIOFILM) of 1 or more, for example between 1 and 10.Modules 40 having sheets 26 woven across the entire length of the fibers10, in a dense weave with a high number of fibers for very highloadings, for example, are useful. For feeds having a CODs of 1000 mg/Lor less, a module 40 may have an SA_(OXYGEN)/SA_(BIOFILM) of between 0.2and 2.5. Modules 40 having sheets woven across the entire length of thefibers but with a less dense weave, or sheets 26 with a central open tow20 area, for example, are useful. For feeds having a CODs of 300 mg/L orless, a module 40 may have an SA_(OXYGEN)/SA_(BIOFILM) of 1 or less, forexample between 1 and 10. Modules 40 with sheets 26 have a central opentow 20 area, or modules 40 of loose tows 20, for example, are useful.

FIG. 19 a shows a bench scale reactor having a module 40 made by potting100 tows 20, each of 96 fibres 10 as shown in FIG. 1, into an opposedpair of headers 44. The module 40 was used to treat a feed water in abatch process. In the process, the module 40 was located in a tank 112filled to 4 L of synthetic wastewater. The tank was drained and filledwith fresh feed every 1 to 7 days. Air was applied to the module at 10mL/min. A biofilm 30 of stable thickness grew on the module 40 for aperiod of over 6 months. The biofilm 30 was essentially endogenous, itsrate of growth generally equal to its rate of decay, except that a smallpart of the biofilm 30 broke off and was discharged with some of thetank drains. A section of a tow 20 is shown in FIG. 19 b. Individualfibers 10 are covered in biofilm 30. In some places, the biofilm 30around a small group of fibers 10 may merge together for a portion ofthe length of the fibers 10. The thickness of the biofilm 30 shown isabout 250 microns.

Referring now to FIG. 20, another reactor is shown as suitable, forexample, for a septic tank, septic tank retrofit or shipboard treatmentplant. The particular reactor shown is a septic tank retrofit using astandard septic tank 410 with an inlet 412 and an outlet 414 on oppositesides. The tank 410 has two stages including a primary chamber 416 and asecondary chamber 418. A dividing wall 420 has a submerged orifice 422that allows flow between the chambers 416, 418. One or more modules 424are placed in the secondary chamber 418. Air is supplied to the bottomheaders of the modules 424 through inlet tubes 426. Exhaust air isvented from the upper headers of the modules 424 through exhaust tubes428. Scouring air is periodically applied to a sparger 430 located underor near the bottom of the modules 424 through scouring air tube 432. Themodules 424 each have 1 to 100 or 8-20 sheets as in FIG. 4 potted into apair of headers to produce a module 424. For example, a septic tank fora single household may have one 8 to 10 sheet module 424 fed with a ¼ hpair blower and creating a pressure drop of about 1 to 7 psi, or about 3psi. With a typical household feed, a generally endogenous biofilm growson the individual fibre 19 and tow 20 surfaces. Biological treatment inthe biofilm results in a reduction in the suspended solids and chemicaloxygen demand of the effluent, allowing the septic tile field to bereduced in size or eliminated.

In another embodiment of the invention, a module 40 may be placed in asuspended growth bioreactor and used in addition to or in place of finebubble aerators to supply substantially molecular oxygen to the mixedliquor to achieve high oxygen transfer efficiency and a high oxygenuptake rate to reduce power use and tank volume. Under conditions of lowsoluble and total substrate concentration that exist in a municipal orindustrial suspended growth bioreactor, biofilm thickness is limited toa very small and stable value by endogenous respiration. Limitedagitation is provided to maintain the biomass in suspension, which mayfurther restrict the growth of the biofilm. Since the oxygen diffusionrate through polyolefin fibre as disclosed in this invention issubstantially lower than oxygen diffusion through the liquid film or abiofilm, the liquid film or biofilm that may grow on the surface of thefibre does not become the mass transfer-controlling step and a highlevel of agitation, high gas pressure or frequent cleaning are notrequired to control the thickness of the biofilm and the liquid boundarylayer. In this invention, the dense or semi-porous nature of the fibrealso prevents wetting, and supply of adequate quantity of oxygen isensure by the very high specific surface area of the fine fibre,removing or reducing the need of very high level of agitation or highgas pressure. Also, the overall oxgen transfer rates are such that anyof air, or oxygen enriched air, or substantially pure oxygen gas may beused as the source of oxygen, resulting in high oxygen transfer andutilization efficiency in each case. In each of these cases, inert gasescan be continuously purged, by having the gas flow through and exhaustfrom the module, to maintain good oxygen concentration profilethroughout the module to ensure good oxygen transfer, although pureoxygen can also be supplied in a dead-end mode.

In another embodiment of the invention, the module 40 may be placed in asuspended growth bioreactor and used for supplying substantiallymolecular oxygen to the mixed liquor to achieve high oxygen transferefficiency and a high oxygen uptake rate to reduce power use and tankvolume. A thin biofilm may be permitted to grow on the surface of thebioreactor. COD or ammonia or both may be removed in this biofilm whichmay be, for example, between 20 and 100 microns thick. In conditions oflow substrate concentration that exist in a suspended growth bioreactor,such biofilm will be enriched in nitrifying microorganisms due to thehigh concentration of oxygen at the fibre surface. This biofilm, whiletransferring adequate oxygen to the mixed liquor, enhances the rate andextent of nitrification in the suspended growth bioreactor by actuallyincreasing the total concentration of nitrifying microorganisms. It alsopromotes better nitrification under conditions, such as hightemperature, where nitrification is severely restricted in a suspendedgrowth bioreactor, due to the stability and robustness of the biofilm,which protects the microorganisms through a thick coating ofexracellular polymeric substances.

In another embodiment of the invention, the module 40 is used to supplyoxygen at a high efficiency to a suspended growth bioreactor incombination with a conventional fine or coarse bubble aerator or anotheraeration device such as a jet aerator. In such an invention, bulk of thebioreactor oxygen demand, typically 80%, but as low as 50% of the total,may be supplied by a gas transfer module, whereas the balance, or 20%,to as high as 50%, is supplied by a conventional aerator which is alsoused for maintaining the biomass in suspension and for helping tocontrol against excessive biofilm growth.

In another embodiment of the invention, the module 40 is used to supplyoxygen at a high efficiency to a deep suspended growth bioreactor,without increasing the inlet oxygen carrying gas pressure to beyond thatrequired for overcoming the pressure drop in the module, as the gas isisolated from the high static pressure in a deep tank due to the densenature of the fibre wall. Mechanical mixing is used to maintain thebiomass suspended in the bioreactor. This facilitates better utilizationof the space available, and reduces the cost of bioreactor because ofits deep profile without incurring a penalty in high air supply pressureas is common in conventional disk, diaphragm, coarse bubble or jetaerators that must deliver oxygen to the bottom of the tank.

FIG. 32 presents a process flow diagram of a membrane aerated suspendedgrowth bioreactor which may be used, with modifications as necessary,with the embodiments discussed above or others in which a module 40 isused to transfer oxygen to a substrate. Raw or pretreated feed enters,through feed line 501, an aerobic bioreactor tank 502 having a membraneaeration zone 503 in which modules 40 (not illustrated) are placed. Air,through module air line 504, is supplied to the modules 40. Suspendedgrowth is maintained in the aerobic bioreactor tank 502 by feeding airthrough mixing air line 505 to a coarse or fine bubble aerator 506.Aerator 506 may receive only a portion of the total air needs of theaerobic bioreactor tank 502, for example in the range of 10 to 50% or 10to 30% and may, or may not, create significant additional oxygentransfer as desired. Alternatively, a mechanical mixer may be used inplace of or in addition to the aerator 506 to keep mixed liquor in theaerobic bioreactor tank 502 mixed and/or keep bacteria in suspension.Mixed liquor leaves the aerobic bioreactor tank 502 through mixed liquorline 510 and enters a separator 509 which may be a clarifier, filter,microfilter or ultrafiltration membrane. Return sludge flows throughrecycled activated sludge line 507 to maintain the desired mixed liquorconcentration in the aerobic bioreactor tank 502. Excess sludge isrejected through waste sludge line 508. Treated effluent leaves througheffluent outlet 506.

The modules 40 may be placed on or near the floor of the aerobicbioreactor tank 502, optionally above the aerators 506 or other mixingdevice. The modules 40 may take up most or all of the floor area and, ifnecessary, may be provided in two or more layers. Multiple layers may berequired, for example, where fibers having low volumetric delivery, suchas unmodified PP fiber, are used. Multiple layers may also be requiredwhere the aerobic bioreactor tank 502 has a shape that is deep relativeto its area. For most bioreactors, which are in the range of 15 to 25feet high, two module layers are likely to be adequate. For example, fora 1 MGD municipal wastewater treatment plant, aerobic bioreactor volumemay be 718 m³. For a 5 m tank height, tank floor area will be 143.5 m².Total oxygen demand for this reactor may be 2153 kg/d. For the moduledescribed in Calculation Example 7, a total of 30 modules will berequired to supply the oxygen demand. Footprint of each module is 7.5m², but placement of modules 40 in a grid with some space betweenadjacent modules 40 may require a total surface area of 230 m².Therefore, two stacks of 15 modules each may be evenly distributed inthe tank to provide uniform distribution. Mixing aerators can beuniformly placed, one underneath each module stack, and the spacebetween the modules 40 can provide downcomer area to ensure uniformdistribution.

In other embodiments, back diffusion, particularly in fibres of lowselectivity, is useful. For example, the modules 40 may be used in waterused to grow fish in a fish farming operation or aquarium either totransfer oxygen to the water, to support a biofilm or both. Even with afibre 10 of low selectivity, the N₂ gradient towards the water that thefish live in is small or even negative. Accordingly, O₂ is added to thewater without causing N₂ saturation in the water even when air is fedinto the modules. The ability to use air, rather than oxygen as requiredwhen bubbles are used, results in a significant cost reduction.Depending on the concentration in the fish water, ammonia mayback-diffuse into the fibre lumens 14 which helps maintain non-toxicammonia levels in the fish water. In these and other applications wherethe water surrounding the modules 40 is aerobic, the entire biofilm maybe maintained under aerobic conditions.

The biofilm, if any, may also be maintained at a thin and stablethickness under endogenous respiration due to the low food concentrationin the water. In a similar application, a module 40 may be used tooxygenate other bodies of water, such as the hypolimnion of a lake, inwhich mixing is not desired and where removal of some gases could bebeneficial, although a more selective membrane may also be used if onlyoxygen transfer is desired.

Back diffusion may also be used in other applications to advantage. Forexample, the module 40 may be used to oxygenate a suspended growthbioreactor at high oxygen transfer efficiency and high oxygen uptakerate due to the molecular nature of the oxygen transferred. The oxygencarrying gas, which may be air, or oxygen enriched air, or substantiallypure oxygen, is supplied to the lumen of the fibre. Oxygen permeatesthrough the fibre wall, and through a thin biofilm that grows on thesurface of the fibre, to the reactor, where it is taken up rapidly bythe active biomass suspended in the mixed liquor. Under such conditionsof high oxygen uptake rate, carbon dioxide will build up in thebioreactor, leading to a decrease in the pH of the mixed liquor whichwould necessitate addition of an alkali to control pH. However, with amodule 40 as described herein, and particularly one of low selectivity,carbon dioxide may diffuse back through the biofilm and the fibre to theoxygen bearing gas stream, leading to reduced or no pH adjustmentrequirement even at oxygen uptake rates as high as 200 mg/L/h.

In another embodiment of the invention, a number of bioreactors areinstalled in series to provide flow patterns approaching plug flow. Thisresults in higher reaction rates and better utilization of oxygen.

In another embodiment of the invention, different oxygen levels are usedin different stages of the bioreactor by oxygen spiking to meetdifferent levels of oxygen demand and to achieve high bioreactorloadings. Different oxygen levels may also be used at different times ina single reactor or stage of a reactor. To increase the oxygen level,the pressure of the gas fed to the lumens of the fibers or the oxygencontent of the feed gas can be increased. Similarly, to decrease theoxygen level, the feed gas pressure or oxygen content can be decreased.Higher oxygen levels may be used in upstream stages of multi-stagereactors or in highly loaded reactors. Oxygen levels may also beincreased periodically or from time to time to correspond to periods oftime when the loading on a reactor is temporarily increased, for exampleto respond to seasonal or daily variations in wastewater strength orquantity.

A module 40 may also be used in alternative processes or arrangements.For example, gas transfer into a liquid can be achieved in a dead endconfiguration, ie. without an outlet conduit. In this case, however, itis preferable to provide a small outlet bleed to reduce condensation inthe lumens and vent gases transferred from the liquid into the lumens.To remove gases from a liquid, a dead end configuration may also be usedwherein no inlet conduit is provided. Use of the apparatus in some otherapplications is described below.

In water degassing, water containing dissolved gases such as nitrogen,oxygen or carbon dioxide flows into a tank. A module 40 is immersed inthe tank. A sweep gas flows through the module 40 or a vacuum is appliedto the module 40 (the inlet conduit is omitted). Gases in the liquidcross the membrane to the lumens of the module 40 from where they areremoved through the outlet conduit. Water lean in dissolved gases leavesthe tank. Such a process is useful, for example, in producing ultrapurewater. Pervaporation is accomplished with a similar reactor but the feedwater contains volatile organic compounds which diffuse to the lumens ofthe module 40.

In humidification, a module 40 is immersed in a water bath. Dry airenters the module 40. Water vapour crosses the membrane to the lumens ofthe module 40 and humid air leaves the module 40.

In air cleaning, a module 40 is immersed in a water bath enriched withnutrients and a biofilm is cultured on the module 40. Air containingvolatile organic compounds flows into the module 40 and the volatileorganic compounds diffuse through the membranes to the biofilm. Air leanin volatile organic compounds exits the module 40.

3.0 Biofilm Control

In a membrane supported biofilm reactor, it can be advantageous tocontrol the thickness of the biofilm on the membranes. For example, inthe reactor 100 (FIG. 17), although the tank 112 is drainedperiodically, most of the biofilm 132 remains on the membranes 120,particularly where the feed has a high COD, for example over 300 mg/L.Excess thickness of the biofilm 132, for example 2 mm thick or more,provides minimal, if any, increase in digestion rate, over a thinnerlayer, for example of 1 mm thick or less. However, keeping the biofilm132 thin allows the sheets 26 of the modules 40 to be placed closertogether, providing more surface area per module volume. This increasein surface area generally more than offsets any minor increase indigestion that may, or may not, be achieved with a thicker biofilm 132.

Accordingly, means are provided to prevent the biofilm 32 from becomingunnecessarily thick. The following methods may be performed individuallyor in various combinations. The frequency of treatment varies with thegrowth rate of the biofilm 132. For example, a biofilm 132 may grow by10 microns a day and the module 40 may be made to tolerate a biofilm ofbetween 0.2 mm and 0.8 mm. Biofilm control procedures may then berequired every 5 to 10 days. Alternately, the period between biofilmcontrol procedures may be linked to the amount of COD that the biofilmhas digested since the last control procedure, which is in turn relatedto the time and biofilm thickness increase since the last controlprocedure. For example, control procedures may be performed when thebiofilm has digested about 20 to 200 grams of CODs per square meter ofbiofilm area since the last control procedure. When control or thicknessreducing procedures are performed at this frequently, a stable biofilmlayer is maintained over extended periods of time even though eachcontrol period does not have a drastic effect on biofilm thickness.Control procedures may be applied to the entire biofilm at once or to aportion of the biofilm at a time.

3.1 Mechanical Methods of Biofilm Control

Some methods for controlling the thickness of the biofilm 132 on themembranes 120 involve mechanically removing part of the biofilm 132. Inone such method, still referring to FIG. 17, one or more aerators 134are provided below the modules 114 and connected to a blower 136 throughan aeration valve 138. With the tank 112 full of liquid, blower 136 isoperated to create bubbles from aerator 134 below the modules 40. Thebubbles mechanically scour the biofilm 132 and also create a flow ofwater through the modules 40 that physically removes some of the biofilm132. A high velocity of scouring air of 2-8 feet/second or an airapplication rate of 5 to 20, for example about 10, cubic meters per hourper square meter of module footprint for intervals of 1 to 10 minutesmay be used. This may be done, for example once every day to once everyweek. Also, air may be used to periodically mix the contents of thebioreactor.

Other mechanical methods include spraying the modules 40 with waterwhile the tank 112 is empty and physically removing biofilm 132 such aswith a comb, wire or brush. The removed biofilm 132 falls to the floorof the tank 112 and may be flushed out through drain 131 for furtherprocessing as for waste sludge. These mechanical methods may beperformed less frequently than other methods and, when performed, may beperformed after another method has weakened the biofilm 132.

Mechanical methods for controlling the biofilm are enhanced by providingthe sheet 26 with a rough or textured surface, the height of the surfaceundulations being in the range of the desired biofilm thickness. Desiredbiofilm thickness may be 200 to 1,000 microns.

3.2 Chemical Methods

In another embodiment, ozone gas, introduced in the fibre lumen is usedto oxidize a part of the biofilm to make it digestible. Oxygen is thenprovided to the lumens to permit the biofilm to digest the oxidizedorganics, thereby reducing the total amounts of solids generated and tocontrol the biofilm thickness. The oxygen may be provided as a separatestep or as part of the regular step of digesting wastewater. The reactormay be treated in this way one module or section at a time.

In another method, a control substance is applied to the tank side ofthe biofilm 132. For example, after the tank 112 is drained, clean waterheated to, for example, 35-55 C, may be pumped into the tank 112 by theliquid pump 144. The heated water is kept in the tank 112 for a periodof time (contact period), for example 3-5 hours, sufficient to kill afraction of the biofilm 132 and dissolve some of the organics that formthe biofilm matrix. The biofilm is also starved to some extent sincefeed has been removed. Oxygen may continue to be applied to the lumensor may be turned off. Air scouring may also be provided during thisperiod to enhance biofilm removal, although it may be more economical tocarry out this operation without air scouring, particularly if theblower 136 and aerator 134 can then be eliminated from the reactor 100entirely. The biofilm 132 is also starved to some extent. After thecontact period, the water is drained through drain valve 131. In anindustrial treatment system, the discharge water will have some COD butthe duration of the contact period can be chosen such that the dischargeis still suitable for discharge to a municipal sewer since most of thekilled organisms will remain in the biofilm 32. During a later part ofthe contact period, the living inner part of the biofilm 32 willbiodegrade the killed organisms. The effect of the heated water, orunheated water, may be enhanced with the addition of chemicals such asacids, for example with a pH between 1 and 6 or between 3 and 3, bases,for example with a pH between 8 and 13 or between 9 and 11, or enzymes.The chemicals and their concentration and contact time are chosen topartially dissolve or weaken some organics that are structural componentof the biofilm but to kill only a fraction of the microorganisms whileleaving the majority behind in an active biofilm for rapid restart ofthe reactor.

In another method, a gaseous control substance is applied to the tankside of the biofilm 132. The gas is applied from gas supply 140 whilethe tank 112 is drained at the end of a batch cycle. Lid 146 is closedso that the gas remains in the tank 112. The gas may be of varioustypes, for example an acid such as chlorine. Alternately, ozone may beused. The primary purpose of the ozone is to break up the cell walls ofthe microorganisms in the biofilm 132 to make it more biodegradable. Theamount of ozone applied would not be sufficient to oxidize more thanabout 5% of the biofilm directly and to kill only a fraction of themicroorganisms present in the biofilm. However, refractory organicmaterial is converted to organic material which is later reduced bybiological oxidation when the tank is refilled. The ozone is generatedin a gas phase (air or oxygen) and is easily dispersed in an empty tank112. The ozone is kept in the tank 112 for a period of time allowing itto be absorbed by the biofilm 132. Redox conditions can be controlled inthe tank 112 while it is drained to promote sludge reduction.Alternating aerobic and anaerobic conditions can be established in thebiofilm 132 by turning the feed to the inlet header 116 on and off whilethe tank 112 is filled with ozone to enhance the effects of the ozone.Killed and partially oxidized organisms remain in the biofilm 132 andare later digested in situ such that excess biomass need not be removedfrom the tank 112 for further treatment. Denitrification may also beimproved because the carbon/nitrogen (C/N) ratio increases. Ozone mayalso be used in this method with membranes 120 that are sensitive toozone since the membranes 120 are protected by the biofilm 32.

3.3 Biological Methods

In another method, worms or other animals or higher life forms are usedin an isolated section of the reactor to digest excess biofilm to reducebio-solids generation. The worms etc. are grown in a separatebioreactor. When desired, the worms etc. are applied to the biofilm byfilling the tank with a liquid suspension or brine containing the wormsetc.

Another method of biofilm control is endogenous respiration. By thismethod, the feed loading applied to the biofilm 132 is kept such thatthe rates of decay of the biofilm 132 equals its rate of growth. Inpractice, the rate of growth may exceed the rate of decay by a smallamount in a batch process because some of the biofilm 132 may detach andleave the tank 12 when it is drained. However, endogenous respirationoccurs most practically without limiting the oxygen supply at lowloading rates and so may be used for feeds with low COD concentrations,for example 1000 mg/L CODs or less or 300 mg/L CODs or less. Endogenousrespiration has the advantage of producing very little waste sludge. Toproduce endogenous respiration without limiting the supply of oxygen tothe biofilm 132, the food to microorganisms ratio (F/M) can be limitedto a value low enough so that the rate of cell growth generally equalsthe rate of cell decay (i.e. a generally null net ratio of cell growth)at an acceptable maximum biofilm thickness. For example, the F/M ratiomay be maintained by applying feed at a rate below 0.09 kg of CODt perday for every kg of MLSS in the reactor (0.09 kgCODt/kgMLSS/d), forexample between 0.03 to 0.09 kgCODt/kgMLSS/d or between 0.07 to 0.09kgCODt/kgMLSS/d.

Alternately, endogenous respiration can be produced by limiting thesupply of oxygen to the biofilm 132, even at F/M ratios that exceedthese values, or at an F/M ratio of greater than 0.07 kgCODt/kgMLSS/d orgreater than 0.9 kgCODt/kgMLSS/d. Further, particularly in a multi-stagereactor or system, both methods can be provided. For example, in asystem having an upstream tank, zone or reactor with effluent fed to adownstream tank, zone or reactor, the upstream biofilm 132 may be keptin endogenous respiration by limiting the oxygen supply to the biofilm132 while the downstream biofilm 132 is kept in endogenous respirationby limiting the F/M ratio. In this way, endogenous respiration can beused to treat concentrated feeds, for example feeds with 300 mg/L CODsor 1000 mg/L CODs or more. Because the feed is only partially treated bythe upstream biofilm 132, the total system becomes like a plug flowreactor, particularly if more than two tanks, zones or reactors areused. Tow or loose tow modules, for example those described in part1.4.1, may be used in one or more or all of the stages or themulti-stage reactor or system.

Another method is periodic starvation. In this method, the feed is keptin the tank 112 for an extended period of time such that the CODconcentration drops to below what it is at the end of a typical batchprocess. The biofilm 132 is not nourished and decays rapidly until thestart of the next batch cycle. The biofilm can also be starve byremoving the feed and filling the tank with clean, for example tap orpotable, water, or by loading the reactor at less than 0.1 kg CODs perkg MLSS per day.

In another method, the supply of gas to the inlet header 116 of themodule 40 is turned on and off cyclically for a period of time. Thevarying supply of oxygen shocks the biofilm 132 and causes increaseddecay. Aerobic and anaerobic areas in the biofilm expand and contractwhile consuming, or being consumed by, the other. Alternately, gasessuch as ozone or chlorine, may be added to the inlet header 116 toenhance the shock.

With chemical or biological biofilm control, closer spacing between thesheets 26, for example 3-4 mm, may be used since hydraulic flow throughthe modules 40 is not required as with air scouring, agitation or otherphysical methods of biofilm removal. Chemical or biological methods arealso useful where sheets 26 or fibers 10 or units 19 are not arranged sothat a flow of scouring air will not reach all parts of the biolfim.Chemical or biological biofilm control methods may also be useful withopen sheets 26 or modules with unsupported or loose fibers 10, fiberunits 19 or tows 20 that would be damaged by air scouring, agitation orphysical methods. Alternately, one or more chemical methods, one or moremechanical methods or one or more biological methods may be combined.

EXAMPLES Example 1 Chemical Oxygen Demand (COD) Reduction in a MembraneSupported Bioreactor

A bench scale bioreactor was made using a module generally as presentedin FIGS. 6-9 except that only a single sheet of the fibres was used. Thelength of the sheet was 0.57 m and height 0.45 m, providing a totalbiofilm area of approximately 0.5 m² assuming a with both sides of sheetavailable for biofilm growth. The ratio of surface area for gas transferto surface area of attached biofilm was between about 5 and 6. Inlet airflow was 25 ml/min at a pressure of 34.5 kPa. Reactor volume was 30 L.Synthetic wastewater with a COD level of 1000 mg/l was introduced in abatch manner periodically. The synthetic wastewater consisted of 1.0 g/Lof soluble peptone and 0.03 g/L of sodium hydrogen phosphate dissolvedin tap water. A series of batch reactions were conducted to determinethe rate of reaction and oxygen transfer efficiency. FIG. 21 presentsthe results of three batch periods: a three day period form day 2 to day5, a three day period from day 6 to day 9 and a one day period from day9 to day 10. It can be seen that 80-90% reduction of COD was obtained ineach of the three-day batch periods. A COD reduction of about 40% wasachieved in the one-day batch period suggesting that the rate of CODreduction is higher while the concentration of wastewater is higher andthat the COD reduction rate levels off as the COD concentration in thebatch decreases with time. Oxygen transfer efficiency during theseseries of tests ranged from 50 to 70%, as measured by the exitconcentration of air.

Example 2 Bench Test With Synthetic Wastewater

A bench scale bioreactor was designed using a single sheet module asdescribed for Example 1. Synthetic wastewater with a COD level of 1000mg/l, as described in Example 1, was introduced and treated by thebiofilm on the module. Rates of COD removal and oxygen transfer and thethickness of the biofilm were calculated or measured and recorded. Forabout the first 21 days, the reactor (which has a 30 L fill volume)drained and re-filled with feed after variable batch periods to keep theCODs in the tank generally between 500 and 1000 mg/L. At day 8 and day16, in addition to emptying the tank and re-filling it with new feed,the module was powerwashed with a water sprayer to remove biofilm. Fromabout day 21 to day 30, the biofilm was subjected to starvation (i.e.the tank was filled with tap, i.e. clean or drinkable, water whileoxygen supply continued to the module) and air scouring treatments. Onabout day 30, the tank was emptied and re-filled with feed. From thenon, the tank was emptied and re-filled with wastewater daily but nobiofilm control steps were taken, to allow the biofilm to grow inthickness and observe the effect and rate of such growth. The results ofthe test are presented in FIG. 21. It can be observed that the CODremoval rate varied between about 19 to 38 grams per square meter perday without being proportional to the biofilm thickness. The oxygentransfer varied between about 10 to 15% grams per square meter per day,also over a relatively wide range of biofilm thickness, namely, fromabout 0.5 mm to over 2.3 mm, at which thickness the measurement devicereached its maximum thickness.

Example 3 Pilot Study With Industrial Wastewater

A small pilot study was conducted using four modules generally as shownin FIGS. 6 to 9. Each module has 6 sheets of fibers and a total planarsurface area, or area of biofilm, of about 3.6 m², and a ratio ofsurface area for gas transfer to surface area of attached biofilm ofbetween about 5 and 6. The modules were installed in a 300 liter tank.The reactor was initially operated with peptone (about 2000 mg/l) andthen peptone added to wastewater in a declining ration to accelerate theinitial growth of biofilm on the sheets but then acclimatize the biofilmto the wastewater. After acclimatizing the biofilm, batch operationswere conducted, filling the tank with industrial wastewater. Thewastewater was drawn from multiple sources in ratios chosen to create anfeed COD of about 3000 mg/l. “Pure” oxygen was supplied to the modulesat a feed pressure of about 5 psi. As shown in FIG. 23, bulk CODsconcentration dropped to less than 1000 mg/l in about 2 to 3 days. Itwas also noted that COD removal rates declined with bulk CODsconcentration in the wastewater, and with time, during each batch.

COD removal rates were calculated at different periods of time duringthe batches corresponding to different concentrations of CODs in thetank. Batches having initial CODs of 5000 mg/l and 7000 mg/l were alsotested to observe the effect of higher initial COD concentrations on CODremoval rate. The results are presented in FIG. 24. As indicated in FIG.24, removal rate was generally higher at higher loadings except that, inthe reactor tested, very high loadings did not always produce very highremoval rates suggesting that one or more of air feed pressure, surfacearea for air transfer to biofilm surface area or total module area wereless than optimum for very high loadings.

The same reactor was used for a series of trials conducted undercontinuous operation. In the trials, HRT and inlet CODs were varied. Thefeed gas was “pure” oxygen at a feed pressure of 5 psi. For each trial,the average inlet CODs, outlet CODs and removal rate, organized by HRTof the trial, are presented in FIG. 25. COD removal rates generallydecreased as HRT increased or as inlet CODs decreased.

The effectiveness of biofilm control procedures were also verified inthe reactor during the batch trials mentioned above. Gentle aeration ofabout 1 scfm/module for 15 seconds every hour was applied, primarily formixing, and more aggressive air scouring of about 4 scfm/module for 2-3minutes every 2-3 days was applied primarily to remove biofilm. Thebiofilm thickness was successfully maintained in a range from about 0.2mm to less than 0.8 mm regardless of the average bulk CODs in thereactor, which varied from about 300 mg/L to about 5,500 mg/L.

Example 4 Pilot Study With Municipal Wastewater

Another pilot study was conducted using two modules as described inExample 3, each having a surface area of about 3.6 m², installed in an85 liter tank. Air was supplied to the modules at a feed pressure ofabout 5 psi. Peptone was added initially to the sewage to accelerate theinitial growth of biofilm on the sheets as described for example 3.Batch operations were conducted, filling the tank with municipalwastewater, screened through a 3 mm screen, having an initial CODs ofaveraging about 100 to 200 mg/l, but occasionally up to 700 mg/L. At theends of the batches, CODs concentration had generally dropped to lessthan 30 mg/l and COD removal rate had also generally dropped to lessthan 1 g/m2/d. The levels of CODs and CODt with respect to time within asample period in a batch are presented in FIG. 26.

A study was also conducted with a continuous process, with differenttrials performed over a total period of about 60 days. In the trials,HRT varied from 24 hours to 3 hours and inlet CODs from 100 mg/l to 200mg/l. Average removal rates tended to be lower with lower loading rates.

Nitrification and denitrification kinetics were also measured in thecontinuous process study. The results of 4 trials are presented in thefollowing table.

TABLE 1 Nitrification and Denitrification in Continuous Operation InletInlet Outlet Outlet Outlet HRT CODs NH3—N CODs NH3—N NO3—N (hr) (mg/L)(mg/L) (mg/L) (mg/L) (mg/L) 11.5 165 18.2 29 3.5 3.4 7.8 117 19.6 25 5.44.4 4.4 105 17.7 35.9 5.6 4.3 3.1 84 18.7 37.6 11.6 1.3

Biofilm control was also tested in the municipal wastewater study.Biofilm thickness averaging 0.2 mm was observed with air scouring, butthicker biofilm appeared to collect between some individual sheetsindicating that these areas were not receiving full scouring air.

Example 5 Bench Scale Study With a Tow Module With Wastewater

A module similar to the one shown in FIG. 5, having 100 PMP fibre tows,each tow having 96 fibres of dense walled PMP, was tested. The totalsurface area of the fibres in the module was 0.54 m². In the module,each tow was individually potted into an upper and lower header. Themodule was fed with a supply of air at a rate of 10 ml/min to the bottomheader and exhausted out of the top header. The module was suspended,with the top header held in a clamp at the water surface and the bottomheader weighed down, in a container filled to a volume of 4 L. Themodule was operated in a batch mode using a synthetic wastewater of 1000mg/L CODs and also wastewater from a septic tank. At the start of eachbatch processing period, the container was filled with wastewater. Airwas supplied to the module to support a biofilm growing on the fibresfor processing periods ranging from between about 1 to 7 days whilewastewater was neither added to nor withdrawn from the tank. Shorterbatch periods were generally used with wastewater having lowerconcentrations of COD. At the end of the processing period, the tank wasdrained. New wastewater was added to start the next processing period.At various times, the module was removed to non-destructively measurethe thickness of the biofilm on them and measurements of the COD in thewastewater were taken.

The thickness measurements from the tests using synthetic wastewater arerecorded in FIG. 27 which shows the thickness of the biofilm on thefibres over the period of 180 days of operation. There was initially nobiofilm but after about 20 or 40 days a biofilm had developed having athickness that generally ranged between about 100 and 300 μm. For mostof the test run, no additional methods were used to control the biofilmthickness and yet the biofilm thickness remained generally stable andacceptable. Small portions of biofilm were observed to be shed from themodule during at least some of the tank draining operations, and biofilmcontrol was otherwise provided by endogenous growth of the biofilm.However, for a period of approximately 15 days, the module was operatedin a starvation mode. In this mode, the tank was filled with tap waterand air feed was continued. The biofilm was reduced in thickness fromabout 250 μm to about 100 μm during the starvation period indicatingthat the starvation period was effective at reducing the thickness ofthe biofilm.

FIGS. 28 and 29 show the removal rate of COD in tests using thesynthetic wastewater. FIG. 28 shows removal rate as a function of timeand FIG. 29 shows removal rate as a function of COD concentration.Referring first to FIG. 28, each vertical line within the figureindicates the start of a new batch processing period. Accordingly, atthe times indicated by the vertical lines, new wastewater having a CODof 1,000 mg/L was added to the tank. As the batch progresses, thewastewater is treated and accordingly its COD concentration reduces. Asshown in FIG. 28, the COD removal rate tended to drop with time in eachbatch processing period suggesting that the removal rate is related tothe COD concentration in the wastewater. Further, the removal rate inthe batch between day 154 and day 159 approached zero indicating thatfurther processing time would have marginal value. In FIG. 29, the CODremoval rate is plotted directly against the average COD concentrationin the wastewater. As indicated in FIG. 29, the relationship between CODremoval rate and COD concentration in the wastewater is nearly linearwith the removal rate being generally proportional to the CODconcentration.

For the tests using septic tank wastewater, the wastewater was takenfrom the second chamber of a septic tank. For one trial, thecharacteristics of the wastewater were as follows:

-   Total Chemical Oxygen Demand (COD_(t)):377 mg/L-   Soluble COD (COD_(s)):199 mg/L-   Ammonia Nitrogen (AN):55.1 mg/L-   Total Suspended Solids (TSS):70 mg/L

The module was operated in a batch mode with batch processing periods ofapproximately 24 hours to simulate actual reaction conditions in aseptic tank. Air was supplied during these periods at the rate givenabove to provide oxygen to the biofilm. After one processing period of22 hours and 35 minutes in duration, a sample of the treated wastewaterwas analyzed and results were as follows:

-   COD_(t):140 mg/L-   COD_(s):73 mg/L-   AN:24.7 mg/L    TSS:1 mg/L

A significant improvement in effluent quality was achieved. Inparticular, a huge reduction in TSS was achieved. By visual observation,a large portion of the TSS removed was in the form of colloidal matter.

FIG. 30 records the results from another trial using septic tankwastewater. The reactor was operated for a two-day batch period withconcentration of CODt, CODs TSS and ammonia nitrogen measured at thebeginning, middle and end of the batch period. For comparison purposes,another sample of wastewater taken from the same septic tank on the sameday was placed in a 500 mL graduated cylinder and monitored as acontrol. After two days of operation, reduction of Total COD (CODt) inthe reactor approached 75 mg/L, with a removal in excess of 70%. TSSdropped from 34 mg/L to almost no appreciable TSS after two days oftreatment. Ammonia was also reduced during this period. During the sameperiod, the control had a less than 40% reduction in COD and an increasein TSS. The batch process and reactor effectively treated the septictank wastewater by removing COD but also removing suspended solids, inpart because of the quiescent nature of the process.

Example 6 Chemical Biofilm Control

A biofilm control study was done using the single sheet reactordescribed in Example 1 with a very thick biofilm on it. At the start ofthe test, the tank was drained and 30 L of sodium hydroxide solution indeionized water at a pH of 9.43 and a temperature of 40 C was added tothe reactor. After a first 4 hours of soak, air scouring at 2 scfm wasstarted and was continued for more than 18 hours while sodium hydroxidesolution remained in the tank. Air supply to the lumens remained on. Thebiofilm thickness was reduced slightly (4.6 mm to 4.3 mm) over the firstfour hour period. After the 18 hours of soaking and air scouring, thethickness of the biofilm was reduced further to 3.2 mm.

In another biofilm control study, 6 single sheet modules as shown inFIGS. 10 a and 10 b were used. Each sheet was about 27 cm long by 20 cmwide and had an available surface area of about 0.11 square meters. Thesheets were woven with the hollow fibers running lengthwise and open atboth ends. The ratio of air transfer area to biofilm area was about 6to 1. The modules were placed in a 20 L (working volume) reactoroperated in batch mode at room temperature with batch periods of about 3days. The reactor was fed with synthetic sewage at concentrations from2000 to 8000 mg/L CODs. Air was fed to the lumens of the modules atabout 2 psi with a flow rate of about 20 mL/min to an inlet header ofeach sheet. At intervals of from 3 to 7 days, between batches, themodules were soaked for 4 hours in a solution of NaOH in hot water witha pH of 10 at 50 C. Air supply to the lumens remained on. After the 4hours, the reactor was re-filled with feed. No air scouring was providedduring the soak periods or during the batch periods. FIG. 31 shows thebiofilm thickness over time which was maintained between 0.2 and 0.8 mmand averaged about 550 microns over a 140 day period. Calculated resultsfrom the batches during that period indicate that during the intervalbetween cleanings the biofilm removed from 66 to 120 grams of CODs persquare meter.

Calculation Example 7 Aerating a Suspended Growth Bioreactor

A module similar to module 40 described above, is used to supplysubstantially molecular oxygen to a suspended growth bioreactor.Important module and process characteristics are as follows:

-   -   1. Fibre material: polymethyl pentene (PMP)    -   2. Oxygen diffusivity in PMP fibre: 2E-13m3/m2/s*m/kPa    -   3. Fibre outside diameter: 68 micron    -   4. Fibre wall thickness: 10 micron    -   5. Total fibre length in the module: 3.78 E+9 cm    -   6. No. of sheets in the module: 297    -   7. Dimension of sheet: 2.3 m long by 1 m high    -   8. Module dimensions: 2.5 m long, by 1 m high by 3 m wide    -   9. Module volume: 7.5 m³    -   10. Effective oxygen transfer area: 6925 m2/module    -   11. Oxygen diffusivity in the biofilm and the liquid boundary        layer: 1.97E-9 cm²/s    -   12. Assumed biofilm and liquid boundary layer thickness: 20        microns    -   13. Oxygen source: air    -   14. Wastewater source: Municipal wastewater    -   15. Bioreactor type: Suspended growth    -   16. Bioreactor bulk oxygen concentration: 2 mg/L    -   17. Inlet gas pressure: 55 kPa gauge    -   18. Outlet gas pressure: 0 kPa gauge

Results are as follows:

-   -   1. Total oxygen transferred through the walls and the biofilm:        70 kg/h    -   2. Oxygen transfer efficiency: 62%    -   3. Volumetric oxygen delivery intensity: 9.3 kg oxygen/m3    -   4. Specific power use: 0.114 kWh/kg oxygen delivered

Comparable calculations for a conventional diaphragm disk operator areas follows:

-   -   1. Oxygen transferred per aerator: 2.93 kg    -   2. Oxygen transfer efficiency: 10%    -   3. Volumetric oxygen delivery intensity (for a 5 m high        bioreactor and limiting oxygen uptake rate to 100 mg O2/L/h: 2.7        kg oxygen/m3    -   4. Specific power use: 0.95 kWh/kg oxygen delivered

It can thus be seen that the invention delivers oxygen to the suspendedgrowth bioreactor at a substantially higher density at a lower energyuse and a high transfer efficiency.

Calculation Example 8 Aerating a Suspended Growth Bioreactor

In this example, the biofilm thickness is increased to 50 micronsinstead of 20 microns.

Performance of this module as an aerator is as follows:

-   -   1. Total oxygen transferred through the walls and the biofilm:        58 kg/h    -   2. Oxygen transfer efficiency: 51%    -   3. Volumetric oxygen delivery intensity: 7.75 kg oxygen/m3    -   4. Specific power use: 0.114 kWh/kg oxygen delivered

Thus it can be seen that while increasing the biofilm thickness reducesefficiency and oxygen delivery, it continues to be far superior to theconventional disk aerator.

Calculation Example 9

In this example, the module described in Calculation Example 7 is used,except the fibre material is PMP with an enhanced permeability which istwice that of PMP. Such enhancement is achieved by using methodsdescribed in this document. To achieve optimum oxygen transfer, inletgas pressure is increased to 151 kPa gauge. Results are as follows:

-   -   1. Total oxygen transferred through the walls and the biofilm:        217 kg/h    -   2. Oxygen transfer efficiency: 51%    -   3. Volumetric oxygen delivery intensity: 29 kg oxygen/m3    -   4. Specific power use: 0.305 kWh/kg oxygen delivered

Thus it can be seen that significant benefits are achieved by enhancingpolymer permeability by methods disclosed in this invention. In thisexample, the air delivery gas pressure is increased to fully utilize theoxygen delivery capacity of the module, although the specific energyrequirement continues to be small compared to the conventional diskaerator. One can as efficiently deliver oxygen by reducing gas pressureto a more common 55 kPa gauge by reducing the length of fibre and themodule.

Calculation Example 10 Aerating a Suspended Growth Bioreactor

In this example, a polypropylene fibre without permeability enhancementis used for making the module described in Calculation Example 7. Thepermeability of PP is only 5.3% of PMP, and this value is used for thecalculation. The module and system described in Calculation Example 1 isused except PP permeability instead of PMP permeability. It isdetermined that the inlet pressure must be dropped to 1.9 kPa gauge toprovide an outlet pressure of 0 kPa gauge. Results are as follows:

-   -   1. Total oxygen transferred through the walls and the biofilm:        2.19 kg/h    -   2. Oxygen transfer efficiency: 72%    -   3. Volumetric oxygen delivery intensity: 0.292 kg oxygen/m3    -   4. Specific power use: 0.022 kWh/kg oxygen delivered

The specific power use is very low, showing that this module can deliverpower very efficiently. However, since an inlet pressure of 1.9 kPa maybe too low to be supplied by existing equipment without significantenergy loss, the sheet length was increased to 11 m in a modified formof this Calculation Example 10. Results are as follows:

-   -   1. Total oxygen transferred through the walls and the biofilm:        16.7 kg/h    -   2. Oxygen transfer efficiency: 72%    -   3. Volumetric oxygen delivery intensity: 2.22 kg oxygen/m3    -   4. Specific power use: 0.099 kWh/kg oxygen delivered

This shows that a PP module without permeability enhancement is superiorin efficiency and specific power use to the conventional disk aeratorand can be used where low capital cost is desirable and power cost ishigh. However, it is preferred that PP permeability be enhanced bymethods disclosed in this patent to enhance the range of applications ofthis invention.

Calculation Example 11

A test was conducted to determine the oxygen transfer efficiency of amodule having sheets, as shown in FIG. 4 a, of polymethyl pentenemembranes. The following two modules were placed in a 106 L reactor:

Description of Modules:

Module 1 Module 2 Module width (m) 0.43 0.23 Fiber length (m) 0.7 0.7Number of fibres 244800 92160 Membrane OD (um) 45 um 45 um Membrane area(m2) 26.5 9.97

The reactor was operated for 20 days with a continuous feed of municipalsewage at an hydraulic retention time of approximately 48 hrs to grow avisible biofilm on the surface of the fibre. The thickness of thebiofilm was difficult to measure but was roughly 50 microns towards theend of the test.

The feed with 0 mg/L oxygen was introduced into the bottom of thereactor and the effluent overflowed to drain.

Experimental Set-Up:

-   Volume of reactor: 106 L-   Reactor hydraulic retention time: 43 hr-   Inlet air pressure: 3.2 psi-   Inlet air flow to modules: 314 ml/min-   Outlet air flow to modules: 280 ml/min-   Inlet oxygen purity: air at 21% by volume-   Outlet oxygen purity: 13.9% by volume-   Oxygen transferred: 58.4 g/d (1.75 g/m2/d)

Substrate inlet and outlet concentrations were as follows:

Parameters Inlet Outlet CODt (mg/L) 588 185 CODs (mg/L) 340 163 TSS(mg/L) 96 40 NH3 (mg/L) 157.5 13 NO3 (mg/L) 1.2 6.9

In addition, total dissolved Oxygen in Bulk (substrate in the reactor)was 5.17 mg/L.

This shows that the system was operating in a hybrid mode, with oxygenpartially being consumed in the biofilm to treat the influent, andpartly being transferred through the biofilm to the bulk, or substrate(partially treated sewage) in the reactor. Therefore, it is concludedthat the invention can be used either to supply oxygen to thebioreactor, or to be used as a hybrid system to support biologicalreaction both in bulk and in the biofilm.

Calculation Example 12

5600 fibres, of a polymethyl pentene textile fibre (outside diameter, 58microns, inside diameter, 41.6 microns) were used to make a module, 1.5m long, with total outside surface area of 1.53 m2. This module was usedto transfer oxygen to mixed liquor from a membrane bioreactor. Mixedliquor was pumped from the bioreactor to a 340 mL oxygen transferreactor. Mixed liquor suspended solids concentration was 13 g/L.Temperature was 20 deg. C. A new module was used with no biofilm grownon it. Operating conditions and oxygen concentrations are as follows:

Oxygen in outlet gas Sludge Air Air flow Dissolved Dissolved streams,flow, pressure, out, oxygen in, oxygen % by mL/min psi L/minute mg/Lout, mg/L volume Water only 10 14 Not Not 19.5 applicable applicable 24010 13.5 0.21 2.89 18.6 370 10 14 .22 1.41 18.7 140 10 14 0.24 3.55 18.7140 5 6 0.22 2.22 18.7

As can be seen, oxygen concentration in the gas stream is reduced andconcentration of dissolved oxygen in the mixed liquor increases. Thisdemonstrates that the dense membrane fibre can be used to transferoxygen to a mixed liquor from a suspended growth bioreactor.

Many modifications and variations of the present invention are possiblewithin the teachings of the invention and the invention may be practicedother than as described above. The scope of the invention is defined bythe following claims.

1. A process for treating a liquid comprising the steps of: a)contacting an apparatus having a port in communication with one or moreinner surfaces of a gas permeable biofilm support medium with theliquid, the medium comprising a membrane having openings of up to about40 Angstroms in size; b) providing a gas to the port of the apparatus,the gas permeating to outer surface(s) of the medium to support abiofilm growing on the outer surface(s); and c) providing air bubbles inthe liquid.
 2. The process of claim 1 wherein the liquid compriseswastewater.
 3. The process of claim 1 wherein the gas comprises oxygen.4. The process of claim 1 performed in a septic tank or shipboard systemor to treat a wastewater taken generally directly from one or morehouses or businesses or parts of a ship.
 5. The process of claim 1wherein the biofilm is maintained at a thickness between 0.05 mm and 2mm, more preferably between 0.1 mm and 1 mm.
 6. A process for treating aliquid comprising the steps of: a) contacting an apparatus having a portin communication with one or more inner surfaces of a gas permeablebiofilm support medium with the liquid; and, b) providing a gas to theport of the apparatus, the gas permeating to outer surface(s) of themedium to support a biofilm growing on the outer surface(s); and, c)maintaining a least a portion of the biofilm so that its thicknessalternately increases and decreases, the biofilm increasing in thicknessover first periods of time and, between the first periods of time,reducing the thickness of the biofilm.
 7. The process of claim 1 whereinstep (b) comprises steps of providing an oxygen bearing gas to the port,the oxygen traveling to the outer surface(s) of the medium and into theliquid.
 8. The process of claim 7 further comprising growing a biofilmon the outer surface(s) of the medium, the oxygen traveling through thebiofilm and into the substrate.
 9. The process of claim 8 furthercomprising maintaining the biofilm under endogenous respiration.
 10. Theprocess of claim 7 wherein the apparatus has an exhaust port, the gas isair, and gases flow through the apparatus from the port to the exhaustport.
 11. The process of claim 10 wherein the exhaust gas has a higherconcentration or mass flow rate of carbon dioxide than the inlet gas.12. The process of claim 8 further comprising performing nitrificationin the biofilm.
 13. The process of claim 7 wherein at least 100 mg/h ofoxygen per liter of substrate is transferred.
 14. The process of claim 8wherein the biofilm has a thickness of at least 20 microns.
 15. Theprocess of claim 8 wherein the biofilm performs one or more of: (a)nitrification (removal of ammonia); and, (b) removal of COD.
 16. Theprocess of claim 8 further comprising maintaining the substrate at a lowsoluble COD concentration and air scrubbing the biofilm to maintain thebiofilm under a certain thickness.
 17. The process of claim 1 whereinthe air bubbles do one or more of (a) add additional oxygen to theliquid; (b) mix the liquid; or, (c) maintain bacteria in suspension inthe liquid.
 18. The process of claim 7 wherein the liquid is waterwithin a suspended growth bioreactor.
 19. The process of claim 7 whereinthe biofilm support medium comprises hollow fibers made of a melt spunthermoplastic polymer subjected to thermal or mechanical treatment afterspinning to have at least an annular region of increased permeability tooxygen without permitting a flow of liquid water through the walls ofthe fibers from the outer surfaces of the fibers to the lumens.
 20. Theprocess of claim 6 wherein the thickness of the biofilm is reduced byproviding the air bubbles in the liquid.
 21. The process of claim 1wherein the air bubbles in the liquid are sufficiently gentle to leavethe biofilm on the outer surface(s) of the medium.
 22. The process ofclaim 1 comprising using the air bubbles in the liquid to maintainbiomass in suspension.
 23. The process of claim 1 wherein the biofilmsupport medium comprises a plurality of membranes, each membranesupporting a layer of biofilm thereon, the air bubbles gently mixing theliquid between adjacent layers of biofilm.